[PATCH v2] gfs2: validate EA record lengths safely during xattr walks

[ZhengYuan Huang <gality369@gmail.com>]

[BUG]
A fuzzed GFS2 image can crash in ea_split_ea() while servicing setxattr:

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

Stack:
 ...
 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:

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

Stack:
 __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 that each EA record stays within the block and
has a valid type, but it does not verify that the recorded span covers
the layout encoded by ea_name_len, ea_data_len, and ea_num_ptrs.
Computing that layout from corrupt metadata must also be overflow-safe.

If a malformed record reaches ea_set_simple() with a recorded length
smaller than its computed layout, 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]
Add an overflow-safe ea_rec_len_valid() helper and call it from
ea_foreach_i(). The helper folds the zero-length check into the common
record-length validation, rejects records whose computed layout
overflows, and rejects records whose recorded span is smaller than the
aligned layout size.

Signed-off-by: ZhengYuan Huang <gality369@gmail.com>
---
v2:
- combine the zero-length and record-size validation in ea_rec_len_valid()
- make the EA layout size calculation overflow-safe
---
 fs/gfs2/xattr.c | 28 +++++++++++++++++++++++++++-
 1 file changed, 27 insertions(+), 1 deletion(-)

diff --git a/fs/gfs2/xattr.c b/fs/gfs2/xattr.c
index b9f48d6f10a9..38f49382c54b 100644
--- a/fs/gfs2/xattr.c
+++ b/fs/gfs2/xattr.c
@@ -8,6 +8,7 @@
 #include <linux/spinlock.h>
 #include <linux/completion.h>
 #include <linux/buffer_head.h>
+#include <linux/overflow.h>
 #include <linux/xattr.h>
 #include <linux/gfs2_ondisk.h>
 #include <linux/posix_acl_xattr.h>
@@ -81,6 +82,31 @@ static bool gfs2_eatype_valid(struct gfs2_sbd *sdp, u8 type)
 	}
 }
 
+static bool ea_rec_len_valid(const struct gfs2_ea_header *ea)
+{
+	size_t size;
+
+	if (!GFS2_EA_REC_LEN(ea))
+		return false;
+
+	size = sizeof(struct gfs2_ea_header) + ea->ea_name_len;
+	if (GFS2_EA_IS_STUFFED(ea)) {
+		if (check_add_overflow(size, (size_t)GFS2_EA_DATA_LEN(ea), &size))
+			return false;
+	} else {
+		size_t ptr_size;
+
+		if (check_mul_overflow(sizeof(__be64), (size_t)ea->ea_num_ptrs,
+				       &ptr_size) ||
+		    check_add_overflow(size, ptr_size, &size))
+			return false;
+	}
+	if (check_add_overflow(size, (size_t)7, &size))
+		return false;
+
+	return GFS2_EA_REC_LEN(ea) >= round_down(size, (size_t)8);
+}
+
 typedef int (*ea_call_t) (struct gfs2_inode *ip, struct buffer_head *bh,
 			  struct gfs2_ea_header *ea,
 			  struct gfs2_ea_header *prev, void *private);
@@ -96,7 +122,7 @@ 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)) {
-		if (!GFS2_EA_REC_LEN(ea)) {
+		if (!ea_rec_len_valid(ea)) {
 			gfs2_consist_inode(ip);
 			return -EIO;
 		}
-- 
2.43.0