[PATCH] mm: Enforce a minimal stack gap even against inaccessible VMAs

[Jann Horn]

As explained in the comment block this change adds, we can't tell what
userspace's intent is when the stack grows towards an inaccessible VMA.

I have a (highly contrived) C testcase for 32-bit x86 userspace with glibc
that mixes malloc(), pthread creation, and recursion in just the right way
such that the main stack overflows into malloc() arena memory.
(Let me know if you want me to share that.)

I don't know of any specific scenario where this is actually exploitable,
but it seems like it could be a security problem for sufficiently unlucky
userspace.

I believe we should ensure that, as long as code is compiled with something
like -fstack-check, a stack overflow in it can never cause the main stack
to overflow into adjacent heap memory.

My fix effectively reverts the behavior for !vma_is_accessible() VMAs to
the behavior before commit 1be7107fbe18 ("mm: larger stack guard gap,
between vmas"), so I think it should be a fairly safe change even in
case A.

Fixes: 561b5e0709e4 ("mm/mmap.c: do not blow on PROT_NONE MAP_FIXED holes in the stack")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Jann Horn <jannh@xxxxxxxxxx>
---
I have tested that Libreoffice still launches after this change, though
I don't know if that means anything.

Note that I haven't tested the growsup code.
---
 mm/mmap.c | 53 ++++++++++++++++++++++++++++++++++++++++++++++-------
 1 file changed, 46 insertions(+), 7 deletions(-)

diff --git a/mm/mmap.c b/mm/mmap.c
index dd4b35a25aeb..971bfd6c1cea 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -1064,10 +1064,12 @@ static int expand_upwards(struct vm_area_struct *vma, unsigned long address)
 		gap_addr = TASK_SIZE;
 
 	next = find_vma_intersection(mm, vma->vm_end, gap_addr);
-	if (next && vma_is_accessible(next)) {
-		if (!(next->vm_flags & VM_GROWSUP))
+	if (next && !(next->vm_flags & VM_GROWSUP)) {
+		/* see comments in expand_downwards() */
+		if (vma_is_accessible(prev))
+			return -ENOMEM;
+		if (address == next->vm_start)
 			return -ENOMEM;
-		/* Check that both stack segments have the same anon_vma? */
 	}
 
 	if (next)
@@ -1155,10 +1157,47 @@ int expand_downwards(struct vm_area_struct *vma, unsigned long address)
 	/* Enforce stack_guard_gap */
 	prev = vma_prev(&vmi);
 	/* Check that both stack segments have the same anon_vma? */
-	if (prev) {
-		if (!(prev->vm_flags & VM_GROWSDOWN) &&
-		    vma_is_accessible(prev) &&
-		    (address - prev->vm_end < stack_guard_gap))
+	if (prev && !(prev->vm_flags & VM_GROWSDOWN) &&
+	    (address - prev->vm_end < stack_guard_gap)) {
+		/*
+		 * If the previous VMA is accessible, this is the normal case
+		 * where the main stack is growing down towards some unrelated
+		 * VMA. Enforce the full stack guard gap.
+		 */
+		if (vma_is_accessible(prev))
+			return -ENOMEM;
+
+		/*
+		 * If the previous VMA is not accessible, we have a problem:
+		 * We can't tell what userspace's intent is.
+		 *
+		 * Case A:
+		 * Maybe userspace wants to use the previous VMA as a
+		 * "guard region" at the bottom of the main stack, in which case
+		 * userspace wants us to grow the stack until it is adjacent to
+		 * the guard region. Apparently some Java runtime environments
+		 * and Rust do that?
+		 * That is kind of ugly, and in that case userspace really ought
+		 * to ensure that the stack is fully expanded immediately, but
+		 * we have to handle this case.
+		 *
+		 * Case B:
+		 * But maybe the previous VMA is entirely unrelated to the stack
+		 * and is only *temporarily* PROT_NONE. For example, glibc
+		 * malloc arenas create a big PROT_NONE region and then
+		 * progressively mark parts of it as writable.
+		 * In that case, we must not let the stack become adjacent to
+		 * the previous VMA. Otherwise, after the region later becomes
+		 * writable, a stack overflow will cause the stack to grow into
+		 * the previous VMA, and we won't have any stack gap to protect
+		 * against this.
+		 *
+		 * As an ugly tradeoff, enforce a single-page gap.
+		 * A single page will hopefully be small enough to not be
+		 * noticed in case A, while providing the same level of
+		 * protection in case B that normal userspace threads get.
+		 */
+		if (address == prev->vm_end)
 			return -ENOMEM;
 	}
 

---
base-commit: 8cf0b93919e13d1e8d4466eb4080a4c4d9d66d7b
change-id: 20241008-stack-gap-inaccessible-c7319f7d4b1b
-- 
Jann Horn <jannh@xxxxxxxxxx>