Patch "mm/mmap: fix __mmap_region() error handling in rare merge failure case" has been added to the 6.12-stable tree

gregkh
2024-11-20T12:31:37Z

This is a note to let you know that I've just added the patch titled

mm/mmap: fix __mmap_region() error handling in rare merge failure case

to the 6.12-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
mm-mmap-fix-__mmap_region-error-handling-in-rare-merge-failure-case.patch
and it can be found in the queue-6.12 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@vger.kernel.org> know about it.

From Liam.Howlett@oracle.com Wed Nov 20 13:30:17 2024
From: "Liam R. Howlett" <Liam.Howlett@oracle.com>
Date: Tue, 19 Nov 2024 12:59:45 -0500
Subject: mm/mmap: fix __mmap_region() error handling in rare merge failure case
To: stable@vger.kernel.org
Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, Andrew Morton <akpm@linux-foundation.org>, Lorenzo Stoakes <lorenzo.stoakes@oracle.com>, Jann Horn <jannh@google.com>, "Liam R. Howlett" <Liam.Howlett@Oracle.com>, syzbot+bc6bfc25a68b7a020ee1@syzkaller.appspotmail.com, Vlastimil Babka <vbabka@suse.cz>
Message-ID: <20241119175945.2600945-1-Liam.Howlett@oracle.com>

From: "Liam R. Howlett" <Liam.Howlett@Oracle.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 is available
in the vma_merge_struct and can be used to detect the -ENOMEM status.

Hitting an -ENOMEM scenario after the driver callback leaves the system
in a state that undoing the mapping is worse than continuing by dipping
into the reserve.

A preallocation should be performed in the case of an -ENOMEM and the
allocations were lost during the failure scenario. The __GFP_NOFAIL
flag is used in the allocation to ensure the allocation succeeds after
implicitly telling the driver that the mapping was happening.

The range is already set in the vma_iter_store() call below, so it is
not necessary and is dropped.

Reported-by: syzbot+bc6bfc25a68b7a020ee1@syzkaller.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@Oracle.com>
Reviewed-by: Vlastimil Babka <vbabka@suse.cz>
Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Jann Horn <jannh@google.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
mm/mmap.c | 13 ++++++++++++-
1 file changed, 12 insertions(+), 1 deletion(-)

--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -1491,7 +1491,18 @@ static unsigned long __mmap_region(struc
vm_flags = vma->vm_flags;
goto file_expanded;
}
- vma_iter_config(&vmi, addr, end);
+
+ /*
+ * In the unlikely even that more memory was needed, but
+ * not available for the vma merge, the vma iterator
+ * will have no memory reserved for the write we told
+ * the driver was happening. To keep up the ruse,
+ * ensure the allocation for the store succeeds.
+ */
+ if (vmg_nomem(&vmg)) {
+ mas_preallocate(&vmi.mas, vma,
+ GFP_KERNEL|__GFP_NOFAIL);
+ }
}

vm_flags = vma->vm_flags;

Patches currently in stable-queue which might be from Liam.Howlett@oracle.com are

queue-6.12/mm-mmap-fix-__mmap_region-error-handling-in-rare-merge-failure-case.patch