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SubjectRe: [2/3] mm: fix up some user-visible effects of the stack guard page
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On Mon, Jan 5, 2015 at 2:21 AM, Jay Foad <jay.foad@gmail.com> wrote:
> Sorry for replying to this old email...
>
>> commit d7824370e26325c881b665350ce64fb0a4fde24a upstream

Heh. From august 2010. That's 4+ years ago.. How come it was noticed
only now? You guys are running excessively old kernels, methinks.

> Address sanitizer tries to find the mapping for the current thread's
> stack by iterating through the entries in /proc/self/maps looking for
> one that contains the address of some random stack variable. This
> fails if the stack mapping has already used up all of its RLIMIT_STACK
> quota, because in that case check_stack_guard_page() will fail to add
> a guard page, but show_map_vma() will still assume that the first page
> of the stack *is* a guard page, and won't report it in /proc/maps.
>
> Here's a small program that demonstrates the failure:

Yup, your analysis sounds correct. My completely untested gut feel is
that the problem is that we don't actually return the error from the
expand_stack() call, so then do_anonymous_page() will allow the extra
guard-page access.

IOW, *maybe* a patch like this. TOTALLY UNTESTED! I may have missed
something, and this may be complete crap.

Linus
include/linux/mm.h | 2 +-
mm/memory.c | 4 ++--
2 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index f80d0194c9bc..80fc92a49649 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -1952,7 +1952,7 @@ extern int expand_downwards(struct vm_area_struct *vma,
#if VM_GROWSUP
extern int expand_upwards(struct vm_area_struct *vma, unsigned long address);
#else
- #define expand_upwards(vma, address) do { } while (0)
+ #define expand_upwards(vma, address) (0)
#endif

/* Look up the first VMA which satisfies addr < vm_end, NULL if none. */
diff --git a/mm/memory.c b/mm/memory.c
index ca920d1fd314..d7e497e98f46 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -2593,7 +2593,7 @@ static inline int check_stack_guard_page(struct vm_area_struct *vma, unsigned lo
if (prev && prev->vm_end == address)
return prev->vm_flags & VM_GROWSDOWN ? 0 : -ENOMEM;

- expand_downwards(vma, address - PAGE_SIZE);
+ return expand_downwards(vma, address - PAGE_SIZE);
}
if ((vma->vm_flags & VM_GROWSUP) && address + PAGE_SIZE == vma->vm_end) {
struct vm_area_struct *next = vma->vm_next;
@@ -2602,7 +2602,7 @@ static inline int check_stack_guard_page(struct vm_area_struct *vma, unsigned lo
if (next && next->vm_start == address + PAGE_SIZE)
return next->vm_flags & VM_GROWSUP ? 0 : -ENOMEM;

- expand_upwards(vma, address + PAGE_SIZE);
+ return expand_upwards(vma, address + PAGE_SIZE);
}
return 0;
}
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