linux-kernel - [PATCH v2] mmap: Fix do_brk_flags() modifying obviously incorrect VMAs

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Date:   Mon, 5 Dec 2022 19:23:17 +0000
From:   Liam Howlett <liam.howlett@...cle.com>
To:     "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>
CC:     Yu Zhao <yuzhao@...gle.com>, Jason Donenfeld <Jason@...c4.com>,
        Matthew Wilcox <willy@...radead.org>,
        SeongJae Park <sj@...nel.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Liam Howlett <liam.howlett@...cle.com>,
        Jann Horn <jannh@...gle.com>
Subject: [PATCH v2] mmap: Fix do_brk_flags() modifying obviously incorrect
 VMAs

Add more sanity checks to the VMA that do_brk_flags() will expand.
Ensure the VMA matches basic merge requirements within the function
before calling can_vma_merge_after().

Drop the duplicate checks from vm_brk_flags() since they will be
enforced later.

Fixes: 2e7ce7d354f2 ("mm/mmap: change do_brk_flags() to expand existing VMA and add do_brk_munmap()")
Suggested-by: Jann Horn <jannh@...gle.com>
Signed-off-by: Liam R. Howlett <Liam.Howlett@...cle.com>
---
 mm/mmap.c | 11 +++--------
 1 file changed, 3 insertions(+), 8 deletions(-)

diff --git a/mm/mmap.c b/mm/mmap.c
index a5eb2f175da0..5d48170fc2b2 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -2946,9 +2946,9 @@ static int do_brk_flags(struct ma_state *mas, struct vm_area_struct *vma,
 	 * Expand the existing vma if possible; Note that singular lists do not
 	 * occur after forking, so the expand will only happen on new VMAs.
 	 */
-	if (vma &&
-	    (!vma->anon_vma || list_is_singular(&vma->anon_vma_chain)) &&
-	    ((vma->vm_flags & ~VM_SOFTDIRTY) == flags)) {
+	if (vma && vma->vm_end == addr && !vma_policy(vma) &&
+	    can_vma_merge_after(vma, flags, NULL, NULL,
+				addr >> PAGE_SHIFT, NULL_VM_UFFD_CTX, NULL)) {
 		mas_set_range(mas, vma->vm_start, addr + len - 1);
 		if (mas_preallocate(mas, vma, GFP_KERNEL))
 			return -ENOMEM;
@@ -3035,11 +3035,6 @@ int vm_brk_flags(unsigned long addr, unsigned long request, unsigned long flags)
 		goto munmap_failed;
 
 	vma = mas_prev(&mas, 0);
-	if (!vma || vma->vm_end != addr || vma_policy(vma) ||
-	    !can_vma_merge_after(vma, flags, NULL, NULL,
-				 addr >> PAGE_SHIFT, NULL_VM_UFFD_CTX, NULL))
-		vma = NULL;
-
 	ret = do_brk_flags(&mas, vma, addr, len, flags);
 	populate = ((mm->def_flags & VM_LOCKED) != 0);
 	mmap_write_unlock(mm);
-- 
2.35.1