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Subject[PATCH 17/19] x86, mpx: rewrite unmap code
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From: Dave Hansen <dave.hansen@linux.intel.com>

The MPX code needs to clear out bounds tables for memory which
is no longer in use. We do this when a userspace mapping is
torn down (unmapped).

There are two modes:
1. An entire bounds table becomes unused, and can be freed
and its pointer removed from the bounds directory. This
happens either when a large mapping is torn down, or when
a small mapping is torn down and it is the last mapping
"covered" by a bounds table.
2. Only part of a bounds table becomes unused, in which case
we free the backing memory as if MADV_DONTNEED was called.

The old code was a spaghetti mess of "edge" bounds tables
where the edges were handled specially, even if we were
unmapping an entire one. Non-edge bounds tables are always
fully unmapped, but share a different code path from the edge
ones. The old code had a bug where it was unmapping too much
memory. I worked on fixing it for two days and gave up.

I didn't write the original code. I didn't particularly like
it, but it worked, so I left it. After my debug session, I
realized it was undebuggagle *and* buggy, so out it went.

I also wrote a new unmapping test program which uncovers bugs
pretty nicely.

Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
---

b/arch/x86/mm/mpx.c | 411 +++++++++++++++++++++-------------------------------
1 file changed, 168 insertions(+), 243 deletions(-)

diff -puN arch/x86/mm/mpx.c~rewrite-unmap-code arch/x86/mm/mpx.c
--- a/arch/x86/mm/mpx.c~rewrite-unmap-code 2015-05-27 09:32:21.211746459 -0700
+++ b/arch/x86/mm/mpx.c 2015-05-27 09:32:21.215746639 -0700
@@ -704,110 +704,6 @@ static int get_bt_addr(struct mm_struct
return 0;
}

-/*
- * Free the backing physical pages of bounds table 'bt_addr'.
- * Assume start...end is within that bounds table.
- */
-static int zap_bt_entries(struct mm_struct *mm,
- unsigned long bt_addr,
- unsigned long start, unsigned long end)
-{
- struct vm_area_struct *vma;
- unsigned long addr, len;
-
- /*
- * Find the first overlapping vma. If vma->vm_start > start, there
- * will be a hole in the bounds table. This -EINVAL return will
- * cause a SIGSEGV.
- */
- vma = find_vma(mm, start);
- if (!vma || vma->vm_start > start)
- return -EINVAL;
-
- /*
- * A NUMA policy on a VM_MPX VMA could cause this bouds table to
- * be split. So we need to look across the entire 'start -> end'
- * range of this bounds table, find all of the VM_MPX VMAs, and
- * zap only those.
- */
- addr = start;
- while (vma && vma->vm_start < end) {
- /*
- * We followed a bounds directory entry down
- * here. If we find a non-MPX VMA, that's bad,
- * so stop immediately and return an error. This
- * probably results in a SIGSEGV.
- */
- if (!is_mpx_vma(vma))
- return -EINVAL;
-
- len = min(vma->vm_end, end) - addr;
- zap_page_range(vma, addr, len, NULL);
- trace_mpx_unmap_zap(addr, addr+len);
-
- vma = vma->vm_next;
- addr = vma->vm_start;
- }
-
- return 0;
-}
-
-static int unmap_single_bt(struct mm_struct *mm,
- long __user *bd_entry, unsigned long bt_addr)
-{
- unsigned long expected_old_val = bt_addr | MPX_BD_ENTRY_VALID_FLAG;
- unsigned long uninitialized_var(actual_old_val);
- int ret;
-
- while (1) {
- int need_write = 1;
- unsigned long cleared_bd_entry = 0;
-
- pagefault_disable();
- ret = mpx_cmpxchg_bd_entry(mm, &actual_old_val,
- bd_entry, expected_old_val, cleared_bd_entry);
- pagefault_enable();
- if (!ret)
- break;
- if (ret == -EFAULT)
- ret = mpx_resolve_fault(bd_entry, need_write);
- /*
- * If we could not resolve the fault, consider it
- * userspace's fault and error out.
- */
- if (ret)
- return ret;
- }
- /*
- * The cmpxchg was performed, check the results.
- */
- if (actual_old_val != expected_old_val) {
- /*
- * Someone else raced with us to unmap the table.
- * There was no bounds table pointed to by the
- * directory, so declare success. Somebody freed
- * it.
- */
- if (!actual_old_val)
- return 0;
- /*
- * Something messed with the bounds directory
- * entry. We hold mmap_sem for read or write
- * here, so it could not be a _new_ bounds table
- * that someone just allocated. Something is
- * wrong, so pass up the error and SIGSEGV.
- */
- return -EINVAL;
- }
-
- /*
- * Note, we are likely being called under do_munmap() already. To
- * avoid recursion, do_munmap() will check whether it comes
- * from one bounds table through VM_MPX flag.
- */
- return do_munmap(mm, bt_addr, mpx_bt_size_bytes(mm));
-}
-
static inline int bt_entry_size_bytes(struct mm_struct *mm)
{
if (is_64bit_mm(mm))
@@ -872,13 +768,69 @@ static inline unsigned long bd_entry_vir
}

/*
- * Return an offset in terms of bytes in to the bounds
- * directory where the bounds directory entry for a given
- * virtual address resides.
- *
- * This has to be in bytes because the directory entries
- * are different sizes on 64/32 bit.
+ * Free the backing physical pages of bounds table 'bt_addr'.
+ * Assume start...end is within that bounds table.
*/
+static noinline int zap_bt_entries_mapping(struct mm_struct *mm,
+ unsigned long bt_addr,
+ unsigned long start_mapping, unsigned long end_mapping)
+{
+ struct vm_area_struct *vma;
+ unsigned long addr, len;
+ unsigned long start;
+ unsigned long end;
+
+ /*
+ * if we 'end' on a boundary, the offset will be 0 which
+ * is not what we want. Back it up a byte to get the
+ * last bt entry. Then once we have the entry itself,
+ * move 'end' back up by the table entry size.
+ */
+ start = bt_addr + mpx_get_bt_entry_offset_bytes(mm, start_mapping);
+ end = bt_addr + mpx_get_bt_entry_offset_bytes(mm, end_mapping - 1);
+ /*
+ * Move end back up by one entry. Among other things
+ * this ensures that it remains page-aligned and does
+ * not screw up zap_page_range()
+ */
+ end += bt_entry_size_bytes(mm);
+
+ /*
+ * Find the first overlapping vma. If vma->vm_start > start, there
+ * will be a hole in the bounds table. This -EINVAL return will
+ * cause a SIGSEGV.
+ */
+ vma = find_vma(mm, start);
+ if (!vma || vma->vm_start > start)
+ return -EINVAL;
+
+ /*
+ * A NUMA policy on a VM_MPX VMA could cause this bounds table to
+ * be split. So we need to look across the entire 'start -> end'
+ * range of this bounds table, find all of the VM_MPX VMAs, and
+ * zap only those.
+ */
+ addr = start;
+ while (vma && vma->vm_start < end) {
+ /*
+ * We followed a bounds directory entry down
+ * here. If we find a non-MPX VMA, that's bad,
+ * so stop immediately and return an error. This
+ * probably results in a SIGSEGV.
+ */
+ if (!is_mpx_vma(vma))
+ return -EINVAL;
+
+ len = min(vma->vm_end, end) - addr;
+ zap_page_range(vma, addr, len, NULL);
+ trace_mpx_unmap_zap(addr, addr+len);
+
+ vma = vma->vm_next;
+ addr = vma->vm_start;
+ }
+ return 0;
+}
+
static unsigned long mpx_get_bd_entry_offset(struct mm_struct *mm,
unsigned long addr)
{
@@ -916,69 +868,80 @@ static unsigned long mpx_get_bd_entry_of
*/
}

-/*
- * If the bounds table pointed by bounds directory 'bd_entry' is
- * not shared, unmap this whole bounds table. Otherwise, only free
- * those backing physical pages of bounds table entries covered
- * in this virtual address region start...end.
- */
-static int unmap_shared_bt(struct mm_struct *mm,
- long __user *bd_entry, unsigned long start,
- unsigned long end, bool prev_shared, bool next_shared)
+static int unmap_entire_bt(struct mm_struct *mm,
+ long __user *bd_entry, unsigned long bt_addr)
{
- unsigned long bt_addr;
- unsigned long start_off, end_off;
+ unsigned long expected_old_val = bt_addr | MPX_BD_ENTRY_VALID_FLAG;
+ unsigned long uninitialized_var(actual_old_val);
int ret;

- ret = get_bt_addr(mm, bd_entry, &bt_addr);
+ while (1) {
+ int need_write = 1;
+ unsigned long cleared_bd_entry = 0;
+
+ pagefault_disable();
+ ret = mpx_cmpxchg_bd_entry(mm, &actual_old_val,
+ bd_entry, expected_old_val, cleared_bd_entry);
+ pagefault_enable();
+ if (!ret)
+ break;
+ if (ret == -EFAULT)
+ ret = mpx_resolve_fault(bd_entry, need_write);
+ /*
+ * If we could not resolve the fault, consider it
+ * userspace's fault and error out.
+ */
+ if (ret)
+ return ret;
+ }
+ /*
+ * The cmpxchg was performed, check the results.
+ */
+ if (actual_old_val != expected_old_val) {
+ /*
+ * Someone else raced with us to unmap the table.
+ * That is OK, since we were both trying to do
+ * the same thing. Declare success.
+ */
+ if (!actual_old_val)
+ return 0;
+ /*
+ * Something messed with the bounds directory
+ * entry. We hold mmap_sem for read or write
+ * here, so it could not be a _new_ bounds table
+ * that someone just allocated. Something is
+ * wrong, so pass up the error and SIGSEGV.
+ */
+ return -EINVAL;
+ }
/*
- * We could see an "error" ret for not-present bounds
- * tables (not really an error), or actual errors, but
- * stop unmapping either way.
+ * Note, we are likely being called under do_munmap() already. To
+ * avoid recursion, do_munmap() will check whether it comes
+ * from one bounds table through VM_MPX flag.
*/
- if (ret)
- return ret;
-
- start_off = mpx_get_bt_entry_offset_bytes(mm, start);
- end_off = mpx_get_bt_entry_offset_bytes(mm, end);
-
- if (prev_shared && next_shared)
- ret = zap_bt_entries(mm, bt_addr,
- bt_addr + start_off,
- bt_addr + end_off);
- else if (prev_shared)
- ret = zap_bt_entries(mm, bt_addr,
- bt_addr + start_off,
- bt_addr + mpx_bt_size_bytes(mm));
- else if (next_shared)
- ret = zap_bt_entries(mm, bt_addr, bt_addr,
- bt_addr + end_off);
- else
- ret = unmap_single_bt(mm, bd_entry, bt_addr);
-
- return ret;
+ return do_munmap(mm, bt_addr, mpx_bt_size_bytes(mm));
}

-/*
- * A virtual address region being munmap()ed might share bounds table
- * with adjacent VMAs. We only need to free the backing physical
- * memory of these shared bounds tables entries covered in this virtual
- * address region.
- */
-static int unmap_edge_bts(struct mm_struct *mm,
- unsigned long start, unsigned long end)
+static int try_unmap_single_bt(struct mm_struct *mm,
+ unsigned long start, unsigned long end)
{
+ struct vm_area_struct *next;
+ struct vm_area_struct *prev;
+ /*
+ * "bta" == Bounds Table Area: the area controlled by the
+ * bounds table that we are unmapping.
+ */
+ unsigned long bta_start_vaddr = start & ~(bd_entry_virt_space(mm)-1);
+ unsigned long bta_end_vaddr = bta_start_vaddr + bd_entry_virt_space(mm);
+ unsigned long uninitialized_var(bt_addr);
+ void __user *bde_vaddr;
int ret;
- long __user *bde_start, *bde_end;
- struct vm_area_struct *prev, *next;
- bool prev_shared = false, next_shared = false;
-
- bde_start = mm->bd_addr + mpx_get_bd_entry_offset(mm, start);
- bde_end = mm->bd_addr + mpx_get_bd_entry_offset(mm, end-1);
-
/*
- * Check whether bde_start and bde_end are shared with adjacent
- * VMAs.
+ * We know 'start' and 'end' lie within an area controlled
+ * by a single bounds table. See if there are any other
+ * VMAs controlled by that bounds table. If there are not
+ * then we can "expand" the are we are unmapping to possibly
+ * cover the entire table.
*
* We already unliked the VMAs from the mm's rbtree so 'start'
* is guaranteed to be in a hole. This gets us the first VMA
@@ -986,102 +949,64 @@ static int unmap_edge_bts(struct mm_stru
* in to 'next'.
*/
next = find_vma_prev(mm, start, &prev);
- if (prev && (mm->bd_addr + mpx_get_bd_entry_offset(mm, prev->vm_end-1))
- == bde_start)
- prev_shared = true;
- if (next && (mm->bd_addr + mpx_get_bd_entry_offset(mm, next->vm_start))
- == bde_end)
- next_shared = true;
-
- /*
- * This virtual address region being munmap()ed is only
- * covered by one bounds table.
- *
- * In this case, if this table is also shared with adjacent
- * VMAs, only part of the backing physical memory of the bounds
- * table need be freeed. Otherwise the whole bounds table need
- * be unmapped.
- */
- if (bde_start == bde_end) {
- return unmap_shared_bt(mm, bde_start, start, end,
- prev_shared, next_shared);
+ if ((!prev || prev->vm_end <= bta_start_vaddr) &&
+ (!next || next->vm_start >= bta_end_vaddr)) {
+ /*
+ * No neighbor VMAs controlled by same bounds
+ * table. Try to unmap the whole thing
+ */
+ start = bta_start_vaddr;
+ end = bta_end_vaddr;
}

+ bde_vaddr = mm->bd_addr + mpx_get_bd_entry_offset(mm, start);
+ ret = get_bt_addr(mm, bde_vaddr, &bt_addr);
/*
- * If more than one bounds tables are covered in this virtual
- * address region being munmap()ed, we need to separately check
- * whether bde_start and bde_end are shared with adjacent VMAs.
+ * No bounds table there, so nothing to unmap.
*/
- ret = unmap_shared_bt(mm, bde_start, start, end, prev_shared, false);
- if (ret)
- return ret;
- ret = unmap_shared_bt(mm, bde_end, start, end, false, next_shared);
+ if (ret == -ENOENT) {
+ ret = 0;
+ return 0;
+ }
if (ret)
return ret;
-
- return 0;
+ /*
+ * We are unmapping an entire table. Either because the
+ * unmap that started this whole process was large enough
+ * to cover an entire table, or that the unmap was small
+ * but was the area covered by a bounds table.
+ */
+ if ((start == bta_start_vaddr) &&
+ (end == bta_end_vaddr))
+ return unmap_entire_bt(mm, bde_vaddr, bt_addr);
+ return zap_bt_entries_mapping(mm, bt_addr, start, end);
}

static int mpx_unmap_tables(struct mm_struct *mm,
unsigned long start, unsigned long end)
{
- int ret;
- long __user *bd_entry, *bde_start, *bde_end;
- unsigned long bt_addr;
-
+ unsigned long one_unmap_start;
trace_mpx_unmap_search(start, end);
- /*
- * "Edge" bounds tables are those which are being used by the region
- * (start -> end), but that may be shared with adjacent areas. If they
- * turn out to be completely unshared, they will be freed. If they are
- * shared, we will free the backing store (like an MADV_DONTNEED) for
- * areas used by this region.
- */
- ret = unmap_edge_bts(mm, start, end);
- switch (ret) {
- /* non-present tables are OK */
- case 0:
- case -ENOENT:
- /* Success, or no tables to unmap */
- break;
- case -EINVAL:
- case -EFAULT:
- default:
- return ret;
- }
-
- /*
- * Only unmap the bounds table that are
- * 1. fully covered
- * 2. not at the edges of the mapping, even if full aligned
- */
- bde_start = mm->bd_addr + mpx_get_bd_entry_offset(mm, start);
- bde_end = mm->bd_addr + mpx_get_bd_entry_offset(mm, end-1);
- for (bd_entry = bde_start + 1; bd_entry < bde_end; bd_entry++) {
- ret = get_bt_addr(mm, bd_entry, &bt_addr);
- switch (ret) {
- case 0:
- break;
- case -ENOENT:
- /* No table here, try the next one */
- continue;
- case -EINVAL:
- case -EFAULT:
- default:
- /*
- * Note: we are being strict here.
- * Any time we run in to an issue
- * unmapping tables, we stop and
- * SIGSEGV.
- */
- return ret;
- }

- ret = unmap_single_bt(mm, bd_entry, bt_addr);
+ one_unmap_start = start;
+ while (one_unmap_start < end) {
+ int ret;
+ unsigned long next_unmap_start = ALIGN(one_unmap_start+1,
+ bd_entry_virt_space(mm));
+ unsigned long one_unmap_end = end;
+ /*
+ * if the end is beyond the current bounds table,
+ * move it back so we only deal with a single one
+ * at a time
+ */
+ if (one_unmap_end > next_unmap_start)
+ one_unmap_end = next_unmap_start;
+ ret = try_unmap_single_bt(mm, one_unmap_start, one_unmap_end);
if (ret)
return ret;
- }

+ one_unmap_start = next_unmap_start;
+ }
return 0;
}

_

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