linux-kernel - [PATCH 6.12.y] mm/mmap: fix __mmap_region() error handling in rare merge failure case

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Message-ID: <20241118194048.2355180-1-Liam.Howlett@oracle.com>
Date: Mon, 18 Nov 2024 14:40:48 -0500
From: "Liam R. Howlett" <Liam.Howlett@...cle.com>
To: Andrew Morton <akpm@...ux-foundation.org>
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
        Jann Horn <jannh@...gle.com>,
        "Liam R. Howlett" <Liam.Howlett@...cle.com>,
        syzbot+bc6bfc25a68b7a020ee1@...kaller.appspotmail.com,
        Vlastimil Babka <vbabka@...e.cz>, stable@...r.kernel.org
Subject: [PATCH 6.12.y] mm/mmap: fix __mmap_region() error handling in rare merge failure case

From: "Liam R. Howlett" <Liam.Howlett@...cle.com>

The mmap_region() function tries to install a new vma, which requires a
pre-allocation for the maple tree write due to the complex locking
scenarios involved.

Recent efforts to simplify the error recovery required the relocation of
the preallocation of the maple tree nodes (via vma_iter_prealloc()
calling mas_preallocate()) higher in the function.

The relocation of the preallocation meant that, if there was a file
associated with the vma and the driver call (mmap_file()) modified the
vma flags, then a new merge of the new vma with existing vmas is
attempted.

During the attempt to merge the existing vma with the new vma, the vma
iterator is used - the same iterator that would be used for the next
write attempt to the tree.  In the event of needing a further allocation
and if the new allocations fails, the vma iterator (and contained maple
state) will cleaned up, including freeing all previous allocations and
will be reset internally.

Upon returning to the __mmap_region() function, the error reason is lost
and the function sets the vma iterator limits, and then tries to
continue to store the new vma using vma_iter_store() - which expects
preallocated nodes.

A preallocation should be performed in case the allocations were lost
during the failure scenario - there is no risk of over allocating.  The
range is already set in the vma_iter_store() call below, so it is not
necessary.

Reported-by: syzbot+bc6bfc25a68b7a020ee1@...kaller.appspotmail.com
Closes: https://syzkaller.appspot.com/x/log.txt?x=17b0ace8580000
Fixes: 5de195060b2e2 ("mm: resolve faulty mmap_region() error path behaviour")
Signed-off-by: Liam R. Howlett <Liam.Howlett@...cle.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Cc: Vlastimil Babka <vbabka@...e.cz>
Cc: Jann Horn <jannh@...gle.com>
Cc: <stable@...r.kernel.org>
---
 mm/mmap.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/mm/mmap.c b/mm/mmap.c
index 79d541f1502b2..5cef9a1981f1b 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -1491,7 +1491,10 @@ static unsigned long __mmap_region(struct file *file, unsigned long addr,
 				vm_flags = vma->vm_flags;
 				goto file_expanded;
 			}
-			vma_iter_config(&vmi, addr, end);
+			if (vma_iter_prealloc(&vmi, vma)) {
+				error = -ENOMEM;
+				goto unmap_and_free_file_vma;
+			}
 		}

 		vm_flags = vma->vm_flags;
-- 
2.43.0