Re: [PATCH v2] mm/vma: fix memory leak in __mmap_region()

Lorenzo Stoakes (Oracle)
2026-03-31T19:24:47Z

On Tue, Mar 31, 2026 at 06:08:11PM +0000, Sechang Lim wrote:
> commit 605f6586ecf7 ("mm/vma: do not leak memory when .mmap_prepare
> swaps the file") handled the success path by skipping get_file() via
> file_doesnt_need_get, but missed the error path.
>
> When /dev/zero is mmap'd with MAP_SHARED, mmap_zero_prepare() calls
> shmem_zero_setup_desc() which allocates a new shmem file to back the
> mapping. If __mmap_new_vma() subsequently fails, this replacement
> file is never fput()'d - the original is released by
> ksys_mmap_pgoff(), but nobody releases the new one.
>
> Add fput() for the swapped file in the error path.
>
> Reproducible with fault injection.
>
> FAULT_INJECTION: forcing a failure.
> name failslab, interval 1, probability 0, space 0, times 1
> CPU: 2 UID: 0 PID: 366 Comm: syz.7.14 Not tainted 7.0.0-rc6 #2 PREEMPT(full)
> Hardware name: QEMU Ubuntu 24.04 PC v2 (i440FX + PIIX, arch_caps fix, 1996), BIOS 1.16.3-debian-1.16.3-2 04/01/2014
> Call Trace:
> <TASK>
> dump_stack_lvl+0x164/0x1f0
> should_fail_ex+0x525/0x650
> should_failslab+0xdf/0x140
> kmem_cache_alloc_noprof+0x78/0x630
> vm_area_alloc+0x24/0x160
> __mmap_region+0xf6b/0x2660
> mmap_region+0x2eb/0x3a0
> do_mmap+0xc79/0x1240
> vm_mmap_pgoff+0x252/0x4c0
> ksys_mmap_pgoff+0xf8/0x120
> __x64_sys_mmap+0x12a/0x190
> do_syscall_64+0xa9/0x580
> entry_SYSCALL_64_after_hwframe+0x76/0x7e
> </TASK>
>
> kmemleak: 1 new suspected memory leaks (see /sys/kernel/debug/kmemleak)
> BUG: memory leak
> unreferenced object 0xffff8881118aca80 (size 360):
> comm "syz.7.14", pid 366, jiffies 4294913255
> hex dump (first 32 bytes):
> 00 00 00 00 ad 4e ad de ff ff ff ff 00 00 00 00 .....N..........
> ff ff ff ff ff ff ff ff c0 28 4d ae ff ff ff ff .........(M.....
> backtrace (crc db0f53bc):
> kmem_cache_alloc_noprof+0x3ab/0x630
> alloc_empty_file+0x5a/0x1e0
> alloc_file_pseudo+0x135/0x220
> __shmem_file_setup+0x274/0x420
> shmem_zero_setup_desc+0x9c/0x170
> mmap_zero_prepare+0x123/0x140
> __mmap_region+0xdda/0x2660
> mmap_region+0x2eb/0x3a0
> do_mmap+0xc79/0x1240
> vm_mmap_pgoff+0x252/0x4c0
> ksys_mmap_pgoff+0xf8/0x120
> __x64_sys_mmap+0x12a/0x190
> do_syscall_64+0xa9/0x580
> entry_SYSCALL_64_after_hwframe+0x76/0x7e
>
> Found by syzkaller.
>
> Fixes: 605f6586ecf7 ("mm/vma: do not leak memory when .mmap_prepare swaps the file")

Sorry I was having a senior moment when I said no cc: Stable, we should have that :)

Andrew could you add:

Cc: <stable@vger.kernel.org>

Here? Thanks!

> Reviewed-by: Lorenzo Stoakes (Oracle) <ljs@kernel.org>
> Signed-off-by: Sechang Lim <rhkrqnwk98@gmail.com>
> -------
> v2:
> - Drop redundant map.file NULL check (Lorenzo)
> - Add comment explaining the fput() (Lorenzo)
> v1: https://lore.kernel.org/linux-mm/20260331121906.1301155-1-rhkrqnwk98@gmail.com/
> mm/vma.c | 7 +++++++
> 1 file changed, 7 insertions(+)
>
> diff --git a/mm/vma.c b/mm/vma.c
> index be64f781a3aa..c8df5f561ad7 100644
> --- a/mm/vma.c
> +++ b/mm/vma.c
> @@ -2781,6 +2781,13 @@ static unsigned long __mmap_region(struct file *file, unsigned long addr,
> if (map.charged)
> vm_unacct_memory(map.charged);
> abort_munmap:
> + /*
> + * This indicates that .mmap_prepare has set a new file, differing from
> + * desc->vm_file. But since we're aborting the operation, only the
> + * original file will be cleaned up. Ensure we clean up both.
> + */
> + if (map.file_doesnt_need_get)
> + fput(map.file);
> vms_abort_munmap_vmas(&map.vms, &map.mas_detach);
> return error;
> }
> --
> 2.43.0
>