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Subject[PATCH] futex: Fix uninterruptble loop due to gate_area
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Subject: futex: Fix uninterruptble loop due to gate_area
From: Peter Zijlstra <a.p.zijlstra@chello.nl>
Date: Fri Dec 02 14:12:06 CET 2011

It was found (by Sasha) that if you use a futex located in the gate
area we get stuck in an uninterruptible infinite loop, much like the
ZERO_PAGE issue.

While looking at this problem, I realized you'll get into similar
trouble when hitting any install_special_pages() mapping. The solution
chosen was not to modify special_mapping_fault() to install a non-zero
page->mapping because that might lead to issues when freeing these
pages. Instead do a find_vma() when we find we're again in the
!mapping branch.

Cc: stable@kernel.org
Reported-by: Sasha Levin <levinsasha928@gmail.com>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
---
include/linux/mm.h | 1 +
kernel/futex.c | 40 +++++++++++++++++++++++++++++++++++-----
mm/mmap.c | 5 +++++
3 files changed, 41 insertions(+), 5 deletions(-)
Index: linux-2.6/include/linux/mm.h
===================================================================
--- linux-2.6.orig/include/linux/mm.h
+++ linux-2.6/include/linux/mm.h
@@ -1394,6 +1394,7 @@ extern int may_expand_vm(struct mm_struc
extern int install_special_mapping(struct mm_struct *mm,
unsigned long addr, unsigned long len,
unsigned long flags, struct page **pages);
+extern bool is_special_mapping(struct vm_area_struct *vma);

extern unsigned long get_unmapped_area(struct file *, unsigned long, unsigned long, unsigned long, unsigned long);

Index: linux-2.6/kernel/futex.c
===================================================================
--- linux-2.6.orig/kernel/futex.c
+++ linux-2.6/kernel/futex.c
@@ -59,6 +59,7 @@
#include <linux/magic.h>
#include <linux/pid.h>
#include <linux/nsproxy.h>
+#include <linux/mm.h>

#include <asm/futex.h>

@@ -236,7 +237,7 @@ get_futex_key(u32 __user *uaddr, int fsh
unsigned long address = (unsigned long)uaddr;
struct mm_struct *mm = current->mm;
struct page *page, *page_head;
- int err, ro = 0;
+ int err, ro = 0, no_mapping_tries = 0;

/*
* The futex address must be "naturally" aligned.
@@ -317,13 +318,42 @@ get_futex_key(u32 __user *uaddr, int fsh
if (!page_head->mapping) {
unlock_page(page_head);
put_page(page_head);
+
/*
- * ZERO_PAGE pages don't have a mapping. Avoid a busy loop
- * trying to find one. RW mapping would have COW'd (and thus
- * have a mapping) so this page is RO and won't ever change.
- */
+ * ZERO_PAGE pages don't have a mapping. Avoid a busy loop
+ * trying to find one. RW mapping would have COW'd (and thus
+ * have a mapping) so this page is RO and won't ever change.
+ */
if ((page_head == ZERO_PAGE(address)))
return -EFAULT;
+
+ /*
+ * Similar problem for the gate area.
+ */
+ if (in_gate_area(mm, address))
+ return -EFAULT;
+
+ /*
+ * There is a special class of pages that will have no mapping
+ * and yet is perfectly valid and not going anywhere. These
+ * are the pages from install_special_mapping(). Since looking
+ * up the vma is expensive, don't do so on the first go round.
+ */
+ if (no_mapping_tries) {
+ struct vm_area_struct *vma;
+
+ err = 0;
+ down_read(&mm->mmap_sem);
+ vma = find_vma(mm, address);
+ if (vma && is_special_mapping(vma))
+ err = -EFAULT;
+ up_read(&mm->mmap_sem);
+
+ if (err)
+ return err;
+ }
+
+ ++no_mapping_tries;
goto again;
}

Index: linux-2.6/mm/mmap.c
===================================================================
--- linux-2.6.orig/mm/mmap.c
+++ linux-2.6/mm/mmap.c
@@ -2479,6 +2479,11 @@ int install_special_mapping(struct mm_st
return ret;
}

+bool is_special_mapping(struct vm_area_struct *vma)
+{
+ return vma->vm_ops == &special_mapping_vmops;
+}
+
static DEFINE_MUTEX(mm_all_locks_mutex);

static void vm_lock_anon_vma(struct mm_struct *mm, struct anon_vma *anon_vma)


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