From a425ac5365f6cb3cc47bf83e6bff0213c10445f7 Mon Sep 17 00:00:00 2001
From: Linus Torvalds <torvalds@linux-foundation.org>
Date: Sun, 25 Jun 2023 14:02:25 -0700
Subject: [PATCH] gup: add warning if some caller would seem to want stack
expansion
It feels very unlikely that anybody would want to do a GUP in an
unmapped area under the stack pointer, but real users sometimes do some
really strange things. So add a (temporary) warning for the case where
a GUP fails and expanding the stack might have made it work.
It's trivial to do the expansion in the caller as part of getting the mm
lock in the first place - see __access_remote_vm() for ptrace, for
example - it's just that it's unnecessarily painful to do it deep in the
guts of the GUP lookup when we might have to drop and re-take the lock.
I doubt anybody actually does anything quite this strange, but let's be
proactive: adding these warnings is simple, and will make debugging it
much easier if they trigger.
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
---
mm/gup.c | 12 ++++++++++--
1 file changed, 10 insertions(+), 2 deletions(-)
diff --git a/mm/gup.c b/mm/gup.c
index e6cdfee4451f0..94102390b273a 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -1096,7 +1096,11 @@ static long __get_user_pages(struct mm_struct *mm,
/* first iteration or cross vma bound */
if (!vma || start >= vma->vm_end) {
- vma = vma_lookup(mm, start);
+ vma = find_vma(mm, start);
+ if (vma && (start < vma->vm_start)) {
+ WARN_ON_ONCE(vma->vm_flags & VM_GROWSDOWN);
+ vma = NULL;
+ }
if (!vma && in_gate_area(mm, start)) {
ret = get_gate_page(mm, start & PAGE_MASK,
gup_flags, &vma,
@@ -1265,9 +1269,13 @@ int fixup_user_fault(struct mm_struct *mm,
fault_flags |= FAULT_FLAG_ALLOW_RETRY | FAULT_FLAG_KILLABLE;
retry:
- vma = vma_lookup(mm, address);
+ vma = find_vma(mm, address);
if (!vma)
return -EFAULT;
+ if (address < vma->vm_start ) {
+ WARN_ON_ONCE(vma->vm_flags & VM_GROWSDOWN);
+ return -EFAULT;
+ }
if (!vma_permits_fault(vma, fault_flags))
return -EFAULT;
--
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