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Subject[PATCH 6/X] uprobes: reimplement xol_add_vma() via install_special_mapping()
I apologize in advance if this was already discussed, but I just can't
understand why xol_add_vma() does not use install_special_mapping().
Unless I missed something this should work and this has the following
advantages:

- we can avoid override_creds() hacks, install_special_mapping()
fools security_file_mmap() passing prot/flags = 0

- no need to play with vma after do_mmap_pgoff()

- no need for get_user_pages(FOLL_WRITE/FOLL_FORCE) hack

- no need for do_munmap() if get_user_pages() fails

- this protects us from mprotect(READ/WRITE)

- this protects from MADV_DONTNEED, the page will be correctly
re-instantiated from area->page

- this makes xol_vma more "cheap", swapper can't see this page
and we avoid the meaningless add_to_swap/pageout.

Note that, before this patch, area->page can't be removed
from the swap cache anyway (we have the reference). And it
must not, uprobes modifies this page directly.

Note on vm_flags:

- we do not use VM_DONTEXPAND, install_special_mapping() adds it

- VM_IO protects from MADV_DOFORK

- I am not sure, may be some archs need VM_READ along with EXEC?

Anything else I have missed?
---
kernel/uprobes.c | 42 +++++++++++++++++++-----------------------
1 files changed, 19 insertions(+), 23 deletions(-)
diff --git a/kernel/uprobes.c b/kernel/uprobes.c
index b59af3b..038f21c 100644
--- a/kernel/uprobes.c
+++ b/kernel/uprobes.c
@@ -1045,53 +1045,49 @@ void munmap_uprobe(struct vm_area_struct *vma)
/* Slot allocation for XOL */
static int xol_add_vma(struct uprobes_xol_area *area)
{
- const struct cred *curr_cred;
struct vm_area_struct *vma;
struct mm_struct *mm;
- unsigned long addr;
+ unsigned long addr_hint;
int ret;

+ area->page = alloc_page(GFP_HIGHUSER);
+ if (!area->page)
+ return -ENOMEM;
+
mm = current->mm;

down_write(&mm->mmap_sem);
ret = -EALREADY;
if (mm->uprobes_xol_area)
goto fail;
-
- ret = -ENOMEM;
/*
* Find the end of the top mapping and skip a page.
- * If there is no space for PAGE_SIZE above
- * that, mmap will ignore our address hint.
- *
- * override credentials otherwise anonymous memory might
- * not be granted execute permission when the selinux
- * security hooks have their way.
+ * If there is no space for PAGE_SIZE above that,
+ * this hint will be ignored.
*/
vma = rb_entry(rb_last(&mm->mm_rb), struct vm_area_struct, vm_rb);
- addr = vma->vm_end + PAGE_SIZE;
- curr_cred = override_creds(&init_cred);
- addr = do_mmap_pgoff(NULL, addr, PAGE_SIZE, PROT_EXEC, MAP_PRIVATE, 0);
- revert_creds(curr_cred);
+ addr_hint = vma->vm_end + PAGE_SIZE;

- if (IS_ERR_VALUE(addr))
+ area->vaddr = get_unmapped_area(NULL, addr_hint, PAGE_SIZE, 0, 0);
+ if (IS_ERR_VALUE(area->vaddr)) {
+ ret = area->vaddr;
goto fail;
+ }

- vma = find_vma(mm, addr);
- /* Don't expand vma on mremap(). */
- vma->vm_flags |= VM_DONTEXPAND | VM_DONTCOPY;
- if (get_user_pages(current, mm, addr, 1, 1, 1,
- &area->page, NULL) != 1) {
- do_munmap(mm, addr, PAGE_SIZE);
+ ret = install_special_mapping(mm, area->vaddr, PAGE_SIZE,
+ VM_EXEC|VM_MAYEXEC | VM_DONTCOPY|VM_IO,
+ &area->page);
+ if (ret)
goto fail;
- }

- area->vaddr = addr;
smp_wmb(); /* pairs with get_uprobes_xol_area() */
mm->uprobes_xol_area = area;
ret = 0;
fail:
up_write(&mm->mmap_sem);
+ if (ret)
+ __free_page(area->page);
+
return ret;
}



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