Re: [PATCH] gfs2: reject undersized EA records during xattr walks

From: ZhengYuan Huang

Date: Tue May 05 2026 - 22:51:02 EST

On Mon, May 4, 2026 at 9:27 PM Andreas Gruenbacher <agruenba@xxxxxxxxxx> wrote:
>
> Hello ZhengYuan Huang,
>
> On Tue, Apr 28, 2026 at 6:05 AM ZhengYuan Huang <gality369@xxxxxxxxx> wrote:
> > [BUG]
> > A fuzzed GFS2 image can crash in `ea_split_ea()` while servicing setxattr:
> >
> > BUG: KASAN: slab-out-of-bounds in ea_split_ea+0x1f3/0x230 fs/gfs2/xattr.c:835
> > Write of size 4 at addr ffff88801550a1c0 by task syz.0.379/950
> >
> > Call Trace:
> > ...
> > ea_split_ea+0x1f3/0x230 fs/gfs2/xattr.c:835
> > ea_set_simple_noalloc fs/gfs2/xattr.c:888 [inline]
> > ea_set_simple+0x66f/0x7c0 fs/gfs2/xattr.c:951
> > ea_foreach_i+0x2a1/0x560 fs/gfs2/xattr.c:112
> > ea_foreach+0x3cd/0x5d0 fs/gfs2/xattr.c:140
> > ea_set_i+0x176/0x3d0 fs/gfs2/xattr.c:1063
> > __gfs2_xattr_set+0x8de/0xc60 fs/gfs2/xattr.c:1215
> > gfs2_xattr_set+0x2ed/0x450 fs/gfs2/xattr.c:1258
> > __vfs_setxattr+0x14f/0x1c0 fs/xattr.c:200
> > __vfs_setxattr_noperm+0x10b/0x5c0 fs/xattr.c:234
> > __vfs_setxattr_locked+0x172/0x240 fs/xattr.c:295
> > vfs_setxattr+0x167/0x390 fs/xattr.c:321
> > do_setxattr+0x13c/0x180 fs/xattr.c:636
> > filename_setxattr+0x16b/0x1c0 fs/xattr.c:665
> > path_setxattrat+0x1d8/0x280 fs/xattr.c:713
> > __do_sys_setxattr fs/xattr.c:747 [inline]
> > __se_sys_setxattr fs/xattr.c:743 [inline]
> > __x64_sys_setxattr+0xcd/0x150 fs/xattr.c:743
> > ...
> >
> > Allocated by task 893:
> > kasan_save_stack+0x39/0x70 mm/kasan/common.c:56
> > kasan_save_track+0x14/0x40 mm/kasan/common.c:77
> > kasan_save_alloc_info+0x37/0x60 mm/kasan/generic.c:573
> > unpoison_slab_object mm/kasan/common.c:342 [inline]
> > __kasan_slab_alloc+0x9d/0xa0 mm/kasan/common.c:368
> > kasan_slab_alloc include/linux/kasan.h:252 [inline]
> > slab_post_alloc_hook mm/slub.c:4978 [inline]
> > slab_alloc_node mm/slub.c:5288 [inline]
> > kmem_cache_alloc_noprof+0x1f0/0x7d0 mm/slub.c:5295
> > __do_sys_getcwd+0xe0/0x730 fs/d_path.c:416
> > __se_sys_getcwd fs/d_path.c:412 [inline]
> > __x64_sys_getcwd+0x59/0x80 fs/d_path.c:412
> > x64_sys_call+0x15ac/0x26a0 arch/x86/include/generated/asm/syscalls_64.h:80
> > do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
> > do_syscall_64+0x93/0xf80 arch/x86/entry/syscall_64.c:94
> > entry_SYSCALL_64_after_hwframe+0x76/0x7e
> >
> > Freed by task 893:
> > kasan_save_stack+0x39/0x70 mm/kasan/common.c:56
> > kasan_save_track+0x14/0x40 mm/kasan/common.c:77
> > __kasan_save_free_info+0x3b/0x60 mm/kasan/generic.c:587
> > kasan_save_free_info mm/kasan/kasan.h:406 [inline]
> > poison_slab_object mm/kasan/common.c:252 [inline]
> > __kasan_slab_free+0x6f/0xa0 mm/kasan/common.c:284
> > kasan_slab_free include/linux/kasan.h:234 [inline]
> > slab_free_hook mm/slub.c:2543 [inline]
> > slab_free mm/slub.c:6642 [inline]
> > kmem_cache_free+0x384/0x7a0 mm/slub.c:6752
> > __do_sys_getcwd+0x3f8/0x730 fs/d_path.c:446
> > __se_sys_getcwd fs/d_path.c:412 [inline]
> > __x64_sys_getcwd+0x59/0x80 fs/d_path.c:412
> > x64_sys_call+0x15ac/0x26a0 arch/x86/include/generated/asm/syscalls_64.h:80
> > do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
> > do_syscall_64+0x93/0xf80 arch/x86/entry/syscall_64.c:94
> > entry_SYSCALL_64_after_hwframe+0x76/0x7e
> >
> > and also as:
> >
> > BUG: KASAN: use-after-free in ea_split_ea+0x1f3/0x230 fs/gfs2/xattr.c:835
> > Write of size 4 at addr ffff888023742118 by task syz.0.27/395
> >
> > Call Trace:
> > __dump_stack lib/dump_stack.c:94 [inline]
> > dump_stack_lvl+0xbe/0x130 lib/dump_stack.c:120
> > print_address_description mm/kasan/report.c:378 [inline]
> > print_report+0xd1/0x650 mm/kasan/report.c:482
> > kasan_report+0xfb/0x140 mm/kasan/report.c:595
> > __asan_report_store4_noabort+0x17/0x30 mm/kasan/report_generic.c:385
> > ea_split_ea+0x1f3/0x230 fs/gfs2/xattr.c:835
> > ea_set_simple_noalloc fs/gfs2/xattr.c:888 [inline]
> > ea_set_simple+0x66f/0x7c0 fs/gfs2/xattr.c:951
> > ea_foreach_i+0x2a1/0x560 fs/gfs2/xattr.c:112
> > ea_foreach+0x334/0x5d0 fs/gfs2/xattr.c:162
> > ea_set_i+0x176/0x3d0 fs/gfs2/xattr.c:1063
> > __gfs2_xattr_set+0x8de/0xc60 fs/gfs2/xattr.c:1215
> > gfs2_xattr_set+0x2ed/0x450 fs/gfs2/xattr.c:1258
> > __vfs_setxattr+0x14f/0x1c0 fs/xattr.c:200
> > __vfs_setxattr_noperm+0x10b/0x5c0 fs/xattr.c:234
> > __vfs_setxattr_locked+0x172/0x240 fs/xattr.c:295
> > vfs_setxattr+0x167/0x390 fs/xattr.c:321
> > do_setxattr+0x13c/0x180 fs/xattr.c:636
> > filename_setxattr+0x16b/0x1c0 fs/xattr.c:665
> > path_setxattrat+0x1d8/0x280 fs/xattr.c:713
> > __do_sys_setxattr fs/xattr.c:747 [inline]
> > __se_sys_setxattr fs/xattr.c:743 [inline]
> > __x64_sys_setxattr+0xcd/0x150 fs/xattr.c:743
> > ...
> >
> > [CAUSE]
> > `ea_foreach_i()` validates only that each EA record has a non-zero
> > `ea_rec_len`, stays within the block, and has a valid type. It never checks
> > that the record span actually covers the layout encoded by
> > `ea_name_len`, `ea_data_len`, and `ea_num_ptrs`.
> >
> > If a malformed record reaches `ea_set_simple()` with
> > `GFS2_EA_REC_LEN(ea) < GFS2_EA_SIZE(ea)`, the unsigned subtraction in the
> > split test underflows and falsely claims there is enough free space.
> > `ea_split_ea()` then writes the new EA header outside the real record.
> >
> > [FIX]
> > Reject EA records whose reserved record length is smaller than their
> > computed layout size in `ea_foreach_i()`, which is the common parse
> > boundary for existing xattr metadata. This stops malformed records before
> > any update path can split or otherwise trust the broken geometry.
> >
> > Signed-off-by: ZhengYuan Huang <gality369@xxxxxxxxx>
> > ---
> > fs/gfs2/xattr.c | 7 +++++++
> > 1 file changed, 7 insertions(+)
> >
> > diff --git a/fs/gfs2/xattr.c b/fs/gfs2/xattr.c
> > index b9f48d6f10a9..16cf31bde99a 100644
> > --- a/fs/gfs2/xattr.c
> > +++ b/fs/gfs2/xattr.c
> > @@ -96,6 +96,8 @@ static int ea_foreach_i(struct gfs2_inode *ip, struct buffer_head *bh,
> > return -EIO;
> >
> > for (ea = GFS2_EA_BH2FIRST(bh);; prev = ea, ea = GFS2_EA2NEXT(ea)) {
> > + u32 ea_size;
> > +
> > if (!GFS2_EA_REC_LEN(ea)) {
> > gfs2_consist_inode(ip);
> > return -EIO;
> > @@ -109,6 +111,11 @@ static int ea_foreach_i(struct gfs2_inode *ip, struct buffer_head *bh,
> > gfs2_consist_inode(ip);
> > return -EIO;
> > }
> > + ea_size = GFS2_EA_SIZE(ea);
> > + if (GFS2_EA_REC_LEN(ea) < ea_size) {
> > + gfs2_consist_inode(ip);
> > + return -EIO;
> > + }
> > error = ea_call(ip, bh, ea, prev, data);
> > if (error)
> > return error;
> > --
> > 2.43.0
> >
>
> the '!GFS2_EA_REC_LEN(ea)' and 'GFS2_EA_REC_LEN(ea) < ea_size' checks
> can be combined here. Also, nothing prevents GFS2_EA_SIZE() from
> overflowing.
>
> Can you fix that?
>
> Thanks,
> Andreas
>

Thanks for the suggestions. I have sent a v2 patch addressing the issues above.

Best regards,
ZhengYuan Huang