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Date: Wed, 23 Mar 2022 20:00:55 +0800 From: yebin <yebin10@...wei.com> To: Jan Kara <jack@...e.cz> CC: <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 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.
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