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Date: Wed, 23 Mar 2022 14:36:53 +0100
From: Jan Kara <jack@...e.cz>
To: yebin <yebin10@...wei.com>
Cc: Jan Kara <jack@...e.cz>, tytso@....edu, adilger.kernel@...ger.ca,
linux-ext4@...r.kernel.org, linux-kernel@...r.kernel.org,
lczerner@...hat.com
Subject: Re: [PATCH -next] ext4: fix use-after-free in ext4_search_dir
On Wed 23-03-22 20:00:55, yebin wrote:
> On 2022/3/23 18:47, Jan Kara wrote:
> > On Wed 23-03-22 11:43:04, Ye Bin wrote:
> > > We got issue as follows:
> > > EXT4-fs (loop0): mounted filesystem without journal. Opts: ,errors=continue
> > > ==================================================================
> > > BUG: KASAN: use-after-free in ext4_search_dir fs/ext4/namei.c:1394 [inline]
> > > BUG: KASAN: use-after-free in search_dirblock fs/ext4/namei.c:1199 [inline]
> > > BUG: KASAN: use-after-free in __ext4_find_entry+0xdca/0x1210 fs/ext4/namei.c:1553
> > > Read of size 1 at addr ffff8881317c3005 by task syz-executor117/2331
> > >
> > > CPU: 1 PID: 2331 Comm: syz-executor117 Not tainted 5.10.0+ #1
> > > Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.14.0-0-g155821a1990b-prebuilt.qemu.org 04/01/2014
> > > Call Trace:
> > > __dump_stack lib/dump_stack.c:83 [inline]
> > > dump_stack+0x144/0x187 lib/dump_stack.c:124
> > > print_address_description+0x7d/0x630 mm/kasan/report.c:387
> > > __kasan_report+0x132/0x190 mm/kasan/report.c:547
> > > kasan_report+0x47/0x60 mm/kasan/report.c:564
> > > ext4_search_dir fs/ext4/namei.c:1394 [inline]
> > > search_dirblock fs/ext4/namei.c:1199 [inline]
> > > __ext4_find_entry+0xdca/0x1210 fs/ext4/namei.c:1553
> > > ext4_lookup_entry fs/ext4/namei.c:1622 [inline]
> > > ext4_lookup+0xb8/0x3a0 fs/ext4/namei.c:1690
> > > __lookup_hash+0xc5/0x190 fs/namei.c:1451
> > > do_rmdir+0x19e/0x310 fs/namei.c:3760
> > > do_syscall_64+0x33/0x40 arch/x86/entry/common.c:46
> > > entry_SYSCALL_64_after_hwframe+0x44/0xa9
> > > RIP: 0033:0x445e59
> > > Code: 4d c7 fb ff c3 66 2e 0f 1f 84 00 00 00 00 00 66 90 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 0f 83 1b c7 fb ff c3 66 2e 0f 1f 84 00 00 00 00
> > > RSP: 002b:00007fff2277fac8 EFLAGS: 00000246 ORIG_RAX: 0000000000000054
> > > RAX: ffffffffffffffda RBX: 0000000000400280 RCX: 0000000000445e59
> > > RDX: 0000000000000000 RSI: 0000000000000000 RDI: 00000000200000c0
> > > RBP: 0000000000000000 R08: 0000000000000000 R09: 0000000000000002
> > > R10: 00007fff2277f990 R11: 0000000000000246 R12: 0000000000000000
> > > R13: 431bde82d7b634db R14: 0000000000000000 R15: 0000000000000000
> > >
> > > The buggy address belongs to the page:
> > > page:0000000048cd3304 refcount:0 mapcount:0 mapping:0000000000000000 index:0x1 pfn:0x1317c3
> > > flags: 0x200000000000000()
> > > raw: 0200000000000000 ffffea0004526588 ffffea0004528088 0000000000000000
> > > raw: 0000000000000001 0000000000000000 00000000ffffffff 0000000000000000
> > > page dumped because: kasan: bad access detected
> > >
> > > Memory state around the buggy address:
> > > ffff8881317c2f00: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
> > > ffff8881317c2f80: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
> > > > ffff8881317c3000: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
> > > ^
> > > ffff8881317c3080: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
> > > ffff8881317c3100: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
> > > ==================================================================
> > >
> > > ext4_search_dir:
> > > ...
> > > de = (struct ext4_dir_entry_2 *)search_buf;
> > > dlimit = search_buf + buf_size;
> > > while ((char *) de < dlimit) {
> > > ...
> > > if ((char *) de + de->name_len <= dlimit &&
> > > ext4_match(dir, fname, de)) {
> > > ...
> > > }
> > > ...
> > > de_len = ext4_rec_len_from_disk(de->rec_len, dir->i_sb->s_blocksize);
> > > if (de_len <= 0)
> > > return -1;
> > > offset += de_len;
> > > de = (struct ext4_dir_entry_2 *) ((char *) de + de_len);
> > > }
> > >
> > > Assume:
> > > de=0xffff8881317c2fff
> > > dlimit=0x0xffff8881317c3000
> > >
> > > If read 'de->name_len' which address is 0xffff8881317c3005, obviously is
> > > out of range, then will trigger use-after-free.
> > > To solve this issue, 'dlimit' must reserve 8 bytes, as we will read
> > > 'de->name_len' to judge if '(char *) de + de->name_len' out of range.
> > >
> > > Signed-off-by: Ye Bin <yebin10@...wei.com>
> > Oh, good catch.
> >
> > > diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
> > > index 3f87cca49f0c..276683f7ab77 100644
> > > --- a/fs/ext4/ext4.h
> > > +++ b/fs/ext4/ext4.h
> > > @@ -2273,6 +2273,10 @@ static inline int ext4_forced_shutdown(struct ext4_sb_info *sbi)
> > > * Structure of a directory entry
> > > */
> > > #define EXT4_NAME_LEN 255
> > > +/*
> > > + * Base length of ext4_dir_entry_2 and ext4_dir_entry exclude name
> > > + */
> > > +#define EXT4_BASE_DIR_LEN 8
> > I'd rather use (sizeof(struct ext4_dir_entry_2) - EXT4_NAME_LEN) here...
> >
> > > struct ext4_dir_entry {
> > > __le32 inode; /* Inode number */
> > > diff --git a/fs/ext4/namei.c b/fs/ext4/namei.c
> > > index e37da8d5cd0c..4739a5aa13aa 100644
> > > --- a/fs/ext4/namei.c
> > > +++ b/fs/ext4/namei.c
> > > @@ -1465,7 +1465,7 @@ int ext4_search_dir(struct buffer_head *bh, char *search_buf, int buf_size,
> > > int de_len;
> > > de = (struct ext4_dir_entry_2 *)search_buf;
> > > - dlimit = search_buf + buf_size;
> > > + dlimit = search_buf + buf_size - EXT4_BASE_DIR_LEN;
> > > while ((char *) de < dlimit) {
> > > /* this code is executed quadratically often */
> > > /* do minimal checking `by hand' */
> > This looks wrong because a bit later we use dlimit to verify
> > de+de->name_len and that can certainly go upto bufsize. You need to modify
> > only the condition in the while loop like:
> >
> > while ((char *) de < dlimit - EXT4_BASE_DIR_LEN) {
> >
> > Honza
> I think 'dlimit' also need to minus EXT4_BASE_DIR_LEN when verify
> 'de+de->name_len' .
> Assume:
> de = 0xffff8881317c2ff7
> dlimit = 0x0xffff8881317c3000
> de->name_len = 8
>
> =>
> de + de->name_len = 0xffff8881317c2fff ( <= dlimit=0x0xffff8881317c3000)
> de->name = 'de' address + EXT4_BASE_DIR_LEN = 0xffff8881317c2ff7 + 8 =
> 0xffff8881317c2fff
> If we read 8 bytes form 0xffff8881317c2fff will read out of range.
Ah, I see. Well, I'd rather modify the condition to look like:
if (de->name + de->name_len <= dlimit &&
ext4_match(dir, fname, de)) {
Because that expresses better what we are checking...
Honza
--
Jan Kara <jack@...e.com>
SUSE Labs, CR
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