[syzbot] [fs?] memory leak in __shmem_file_setup

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: f0b9d8eb98df Merge tag 'nfsd-6.19-3' of git://git.kernel.o..
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=12ec819a580000
kernel config: https://syzkaller.appspot.com/x/.config?x=d60836e327fd6756
dashboard link: https://syzkaller.appspot.com/bug?extid=bf5de69ebb4bdf86f59f
compiler: gcc (Debian 12.2.0-14+deb12u1) 12.2.0, GNU ld (GNU Binutils for Debian) 2.40
syz repro: https://syzkaller.appspot.com/x/repro.syz?x=16ec819a580000
C reproducer: https://syzkaller.appspot.com/x/repro.c?x=11bcc19a580000

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/aad2d47ff01d/disk-f0b9d8eb.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/c31e7ae85c07/vmlinux-f0b9d8eb.xz
kernel image: https://storage.googleapis.com/syzbot-assets/5525fab81561/bzImage-f0b9d8eb.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+bf5de6...@syzkaller.appspotmail.com

2026/01/08 07:49:49 executed programs: 5
BUG: memory leak
unreferenced object 0xffff888112c4b240 (size 184):
comm "syz.0.17", pid 6070, jiffies 4294944898
hex dump (first 32 bytes):
00 00 00 00 07 00 0e 02 00 e4 66 85 ff ff ff ff ..........f.....
98 38 89 09 81 88 ff ff 00 00 00 00 00 00 00 00 .8..............
backtrace (crc 987747be):
kmemleak_alloc_recursive include/linux/kmemleak.h:44 [inline]
slab_post_alloc_hook mm/slub.c:4958 [inline]
slab_alloc_node mm/slub.c:5263 [inline]
kmem_cache_alloc_noprof+0x3b4/0x590 mm/slub.c:5270
alloc_empty_file+0x51/0x1a0 fs/file_table.c:237
alloc_file fs/file_table.c:354 [inline]
alloc_file_pseudo+0xae/0x140 fs/file_table.c:383
__shmem_file_setup+0x11a/0x210 mm/shmem.c:5846
shmem_kernel_file_setup mm/shmem.c:5865 [inline]
__shmem_zero_setup mm/shmem.c:5905 [inline]
shmem_zero_setup_desc+0x33/0x90 mm/shmem.c:5936
mmap_zero_prepare+0x4e/0x60 drivers/char/mem.c:524
vfs_mmap_prepare include/linux/fs.h:2058 [inline]
call_mmap_prepare mm/vma.c:2596 [inline]
__mmap_region+0x8b8/0x13e0 mm/vma.c:2692
mmap_region+0x19f/0x1e0 mm/vma.c:2786
do_mmap+0x6a3/0xb60 mm/mmap.c:558
vm_mmap_pgoff+0x1a6/0x2d0 mm/util.c:581
ksys_mmap_pgoff+0x233/0x2d0 mm/mmap.c:604
__do_sys_mmap arch/x86/kernel/sys_x86_64.c:89 [inline]
__se_sys_mmap arch/x86/kernel/sys_x86_64.c:82 [inline]
__x64_sys_mmap+0x6f/0xa0 arch/x86/kernel/sys_x86_64.c:82
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0xa4/0xf80 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f

BUG: memory leak
unreferenced object 0xffff888101e46ca8 (size 40):
comm "syz.0.17", pid 6070, jiffies 4294944898
hex dump (first 32 bytes):
ff ff 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
00 00 00 00 00 00 00 00 f8 52 86 00 81 88 ff ff .........R......
backtrace (crc 2d2a393c):
kmemleak_alloc_recursive include/linux/kmemleak.h:44 [inline]
slab_post_alloc_hook mm/slub.c:4958 [inline]
slab_alloc_node mm/slub.c:5263 [inline]
kmem_cache_alloc_noprof+0x3b4/0x590 mm/slub.c:5270
lsm_file_alloc security/security.c:169 [inline]
security_file_alloc+0x30/0x240 security/security.c:2380
init_file+0x3e/0x160 fs/file_table.c:159
alloc_empty_file+0x6f/0x1a0 fs/file_table.c:241
alloc_file fs/file_table.c:354 [inline]
alloc_file_pseudo+0xae/0x140 fs/file_table.c:383
__shmem_file_setup+0x11a/0x210 mm/shmem.c:5846
shmem_kernel_file_setup mm/shmem.c:5865 [inline]
__shmem_zero_setup mm/shmem.c:5905 [inline]
shmem_zero_setup_desc+0x33/0x90 mm/shmem.c:5936
mmap_zero_prepare+0x4e/0x60 drivers/char/mem.c:524
vfs_mmap_prepare include/linux/fs.h:2058 [inline]
call_mmap_prepare mm/vma.c:2596 [inline]
__mmap_region+0x8b8/0x13e0 mm/vma.c:2692
mmap_region+0x19f/0x1e0 mm/vma.c:2786
do_mmap+0x6a3/0xb60 mm/mmap.c:558
vm_mmap_pgoff+0x1a6/0x2d0 mm/util.c:581
ksys_mmap_pgoff+0x233/0x2d0 mm/mmap.c:604
__do_sys_mmap arch/x86/kernel/sys_x86_64.c:89 [inline]
__se_sys_mmap arch/x86/kernel/sys_x86_64.c:82 [inline]
__x64_sys_mmap+0x6f/0xa0 arch/x86/kernel/sys_x86_64.c:82
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0xa4/0xf80 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f

BUG: memory leak
unreferenced object 0xffff888108f03840 (size 184):
comm "syz-executor", pid 5988, jiffies 4294944899
hex dump (first 32 bytes):
01 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
backtrace (crc 5869ffdf):
kmemleak_alloc_recursive include/linux/kmemleak.h:44 [inline]
slab_post_alloc_hook mm/slub.c:4958 [inline]
slab_alloc_node mm/slub.c:5263 [inline]
kmem_cache_alloc_noprof+0x3b4/0x590 mm/slub.c:5270
prepare_creds+0x22/0x5e0 kernel/cred.c:185
copy_creds+0x44/0x290 kernel/cred.c:286
copy_process+0x979/0x2860 kernel/fork.c:2086
kernel_clone+0x119/0x6c0 kernel/fork.c:2651
__do_sys_clone+0x7b/0xb0 kernel/fork.c:2792
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0xa4/0xf80 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f

BUG: memory leak
unreferenced object 0xffff888109a7b8e0 (size 32):
comm "syz-executor", pid 5988, jiffies 4294944899
hex dump (first 32 bytes):
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
f8 52 86 00 81 88 ff ff 00 00 00 00 00 00 00 00 .R..............
backtrace (crc 336e1c5f):
kmemleak_alloc_recursive include/linux/kmemleak.h:44 [inline]
slab_post_alloc_hook mm/slub.c:4958 [inline]
slab_alloc_node mm/slub.c:5263 [inline]
__do_kmalloc_node mm/slub.c:5656 [inline]
__kmalloc_noprof+0x3e0/0x660 mm/slub.c:5669
kmalloc_noprof include/linux/slab.h:961 [inline]
kzalloc_noprof include/linux/slab.h:1094 [inline]
lsm_blob_alloc+0x4d/0x70 security/security.c:192
lsm_cred_alloc security/security.c:209 [inline]
security_prepare_creds+0x2f/0x270 security/security.c:2763
prepare_creds+0x385/0x5e0 kernel/cred.c:215
copy_creds+0x44/0x290 kernel/cred.c:286
copy_process+0x979/0x2860 kernel/fork.c:2086
kernel_clone+0x119/0x6c0 kernel/fork.c:2651
__do_sys_clone+0x7b/0xb0 kernel/fork.c:2792
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0xa4/0xf80 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f

BUG: memory leak
unreferenced object 0xffff888109b169c0 (size 184):
comm "syz.0.18", pid 6072, jiffies 4294944899
hex dump (first 32 bytes):
00 00 00 00 07 00 0e 02 00 e4 66 85 ff ff ff ff ..........f.....
68 e6 05 0e 81 88 ff ff 00 00 00 00 00 00 00 00 h...............
backtrace (crc 86e9bbaa):
kmemleak_alloc_recursive include/linux/kmemleak.h:44 [inline]
slab_post_alloc_hook mm/slub.c:4958 [inline]
slab_alloc_node mm/slub.c:5263 [inline]
kmem_cache_alloc_noprof+0x3b4/0x590 mm/slub.c:5270
alloc_empty_file+0x51/0x1a0 fs/file_table.c:237
alloc_file fs/file_table.c:354 [inline]
alloc_file_pseudo+0xae/0x140 fs/file_table.c:383
__shmem_file_setup+0x11a/0x210 mm/shmem.c:5846
shmem_kernel_file_setup mm/shmem.c:5865 [inline]
__shmem_zero_setup mm/shmem.c:5905 [inline]
shmem_zero_setup_desc+0x33/0x90 mm/shmem.c:5936
mmap_zero_prepare+0x4e/0x60 drivers/char/mem.c:524
vfs_mmap_prepare include/linux/fs.h:2058 [inline]
call_mmap_prepare mm/vma.c:2596 [inline]
__mmap_region+0x8b8/0x13e0 mm/vma.c:2692
mmap_region+0x19f/0x1e0 mm/vma.c:2786
do_mmap+0x6a3/0xb60 mm/mmap.c:558
vm_mmap_pgoff+0x1a6/0x2d0 mm/util.c:581
ksys_mmap_pgoff+0x233/0x2d0 mm/mmap.c:604
__do_sys_mmap arch/x86/kernel/sys_x86_64.c:89 [inline]
__se_sys_mmap arch/x86/kernel/sys_x86_64.c:82 [inline]
__x64_sys_mmap+0x6f/0xa0 arch/x86/kernel/sys_x86_64.c:82
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0xa4/0xf80 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f

connection error: failed to recv *flatrpc.ExecutorMessageRawT: EOF


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[Lorenzo Stoakes]

Hi all,

I have bisected this to commit ab04945f91bc ("mm: update mem char driver to use
mmap_prepare"), i.e. my patch, so apologies for that.

Will figure out what's happening here and come up with a hotfix.

When I saw /dev/zero I did suspect this exact commit, would have saved me some
bisecting had I just tested it first but there we are :P

Cheers, Lorenzo