[PATCH 6.3 24/29] gup: add warning if some caller would seem to want stack expansion

Greg Kroah-Hartman
2023-06-29T18:46:56Z

From: Linus Torvalds <torvalds@linux-foundation.org>

commit a425ac5365f6cb3cc47bf83e6bff0213c10445f7 upstream.

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
mm/gup.c | 12 ++++++++++--
1 file changed, 10 insertions(+), 2 deletions(-)

--- a/mm/gup.c
+++ b/mm/gup.c
@@ -1096,7 +1096,11 @@ static long __get_user_pages(struct mm_s

/* 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 *m
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;