* Re: KCSAN: data-race in fat16_ent_put / fat_search_long [not found] <00000000000016a19d0596980568@google.com> @ 2019-11-05 14:39 ` Marco Elver 2019-11-05 15:25 ` Matthew Wilcox 0 siblings, 1 reply; 4+ messages in thread From: Marco Elver @ 2019-11-05 14:39 UTC (permalink / raw) To: syzbot; +Cc: hirofumi, linux-kernel, syzkaller-bugs, linux-fsdevel, viro On Tue, 05 Nov 2019, syzbot wrote: > Hello, > > syzbot found the following crash on: > > HEAD commit: 05f22368 x86, kcsan: Enable KCSAN for x86 > git tree: https://github.com/google/ktsan.git kcsan > console output: https://syzkaller.appspot.com/x/log.txt?x=14ccaac8e00000 > kernel config: https://syzkaller.appspot.com/x/.config?x=87d111955f40591f > dashboard link: https://syzkaller.appspot.com/bug?extid=11010f0000e50c63c2cc > compiler: gcc (GCC) 9.0.0 20181231 (experimental) > > Unfortunately, I don't have any reproducer for this crash yet. > > IMPORTANT: if you fix the bug, please add the following tag to the commit: > Reported-by: syzbot+11010f0000e50c63c2cc@syzkaller.appspotmail.com > > ================================================================== > BUG: KCSAN: data-race in fat16_ent_put / fat_search_long > > write to 0xffff8880a209c96a of 2 bytes by task 11985 on cpu 0: > fat16_ent_put+0x5b/0x90 fs/fat/fatent.c:181 > fat_ent_write+0x6d/0xf0 fs/fat/fatent.c:415 > fat_chain_add+0x34e/0x400 fs/fat/misc.c:130 > fat_add_cluster+0x92/0xd0 fs/fat/inode.c:112 > __fat_get_block fs/fat/inode.c:154 [inline] > fat_get_block+0x3ae/0x4e0 fs/fat/inode.c:189 > __block_write_begin_int+0x2ea/0xf20 fs/buffer.c:1968 > __block_write_begin fs/buffer.c:2018 [inline] > block_write_begin+0x77/0x160 fs/buffer.c:2077 > cont_write_begin+0x3d6/0x670 fs/buffer.c:2426 > fat_write_begin+0x72/0xc0 fs/fat/inode.c:235 > pagecache_write_begin+0x6b/0x90 mm/filemap.c:3148 > cont_expand_zero fs/buffer.c:2353 [inline] > cont_write_begin+0x17a/0x670 fs/buffer.c:2416 > fat_write_begin+0x72/0xc0 fs/fat/inode.c:235 > pagecache_write_begin+0x6b/0x90 mm/filemap.c:3148 > generic_cont_expand_simple+0xb0/0x120 fs/buffer.c:2317 > > read to 0xffff8880a209c96b of 1 bytes by task 11990 on cpu 1: > fat_search_long+0x20a/0xc60 fs/fat/dir.c:484 > vfat_find+0xc1/0xd0 fs/fat/namei_vfat.c:698 > vfat_lookup+0x75/0x350 fs/fat/namei_vfat.c:712 > lookup_open fs/namei.c:3203 [inline] > do_last fs/namei.c:3314 [inline] > path_openat+0x15b6/0x36e0 fs/namei.c:3525 > do_filp_open+0x11e/0x1b0 fs/namei.c:3555 > do_sys_open+0x3b3/0x4f0 fs/open.c:1097 > __do_sys_open fs/open.c:1115 [inline] > __se_sys_open fs/open.c:1110 [inline] > __x64_sys_open+0x55/0x70 fs/open.c:1110 > do_syscall_64+0xcc/0x370 arch/x86/entry/common.c:290 > entry_SYSCALL_64_after_hwframe+0x44/0xa9 > > Reported by Kernel Concurrency Sanitizer on: > CPU: 1 PID: 11990 Comm: syz-executor.2 Not tainted 5.4.0-rc3+ #0 > Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS > Google 01/01/2011 > ================================================================== I was trying to understand what is happening here, but fail to see how this can happen. So it'd be good if somebody who knows this code can explain. We are quite positive this is not a false positive, given the addresses accessed match. The two bits of code in question here are: static void fat16_ent_put(struct fat_entry *fatent, int new) { if (new == FAT_ENT_EOF) new = EOF_FAT16; *fatent->u.ent16_p = cpu_to_le16(new); <<== data race here mark_buffer_dirty_inode(fatent->bhs[0], fatent->fat_inode); } int fat_search_long(struct inode *inode, const unsigned char *name, int name_len, struct fat_slot_info *sinfo) { struct super_block *sb = inode->i_sb; struct msdos_sb_info *sbi = MSDOS_SB(sb); struct buffer_head *bh = NULL; struct msdos_dir_entry *de; unsigned char nr_slots; wchar_t *unicode = NULL; unsigned char bufname[FAT_MAX_SHORT_SIZE]; loff_t cpos = 0; int err, len; err = -ENOENT; while (1) { if (fat_get_entry(inode, &cpos, &bh, &de) == -1) goto end_of_dir; parse_record: nr_slots = 0; if (de->name[0] == DELETED_FLAG) continue; if (de->attr != ATTR_EXT && (de->attr & ATTR_VOLUME)) <<== data race here continue; if (de->attr != ATTR_EXT && IS_FREE(de->name)) continue; <snip> } Thanks, -- Marco ^ permalink raw reply [flat|nested] 4+ messages in thread
* Re: KCSAN: data-race in fat16_ent_put / fat_search_long 2019-11-05 14:39 ` KCSAN: data-race in fat16_ent_put / fat_search_long Marco Elver @ 2019-11-05 15:25 ` Matthew Wilcox 2019-11-06 8:31 ` OGAWA Hirofumi 0 siblings, 1 reply; 4+ messages in thread From: Matthew Wilcox @ 2019-11-05 15:25 UTC (permalink / raw) To: Marco Elver Cc: syzbot, hirofumi, linux-kernel, syzkaller-bugs, linux-fsdevel, viro On Tue, Nov 05, 2019 at 03:39:23PM +0100, Marco Elver wrote: > On Tue, 05 Nov 2019, syzbot wrote: > > ================================================================== > > BUG: KCSAN: data-race in fat16_ent_put / fat_search_long > > > > write to 0xffff8880a209c96a of 2 bytes by task 11985 on cpu 0: > > fat16_ent_put+0x5b/0x90 fs/fat/fatent.c:181 > > fat_ent_write+0x6d/0xf0 fs/fat/fatent.c:415 > > fat_chain_add+0x34e/0x400 fs/fat/misc.c:130 > > fat_add_cluster+0x92/0xd0 fs/fat/inode.c:112 > > __fat_get_block fs/fat/inode.c:154 [inline] > > fat_get_block+0x3ae/0x4e0 fs/fat/inode.c:189 > > __block_write_begin_int+0x2ea/0xf20 fs/buffer.c:1968 > > __block_write_begin fs/buffer.c:2018 [inline] > > block_write_begin+0x77/0x160 fs/buffer.c:2077 > > cont_write_begin+0x3d6/0x670 fs/buffer.c:2426 > > fat_write_begin+0x72/0xc0 fs/fat/inode.c:235 > > pagecache_write_begin+0x6b/0x90 mm/filemap.c:3148 > > cont_expand_zero fs/buffer.c:2353 [inline] > > cont_write_begin+0x17a/0x670 fs/buffer.c:2416 > > fat_write_begin+0x72/0xc0 fs/fat/inode.c:235 > > pagecache_write_begin+0x6b/0x90 mm/filemap.c:3148 > > generic_cont_expand_simple+0xb0/0x120 fs/buffer.c:2317 > > > > read to 0xffff8880a209c96b of 1 bytes by task 11990 on cpu 1: > > fat_search_long+0x20a/0xc60 fs/fat/dir.c:484 > > vfat_find+0xc1/0xd0 fs/fat/namei_vfat.c:698 > > vfat_lookup+0x75/0x350 fs/fat/namei_vfat.c:712 > > lookup_open fs/namei.c:3203 [inline] > > do_last fs/namei.c:3314 [inline] > > path_openat+0x15b6/0x36e0 fs/namei.c:3525 > > do_filp_open+0x11e/0x1b0 fs/namei.c:3555 > > do_sys_open+0x3b3/0x4f0 fs/open.c:1097 > > __do_sys_open fs/open.c:1115 [inline] > > __se_sys_open fs/open.c:1110 [inline] > > __x64_sys_open+0x55/0x70 fs/open.c:1110 > > do_syscall_64+0xcc/0x370 arch/x86/entry/common.c:290 > > entry_SYSCALL_64_after_hwframe+0x44/0xa9 > > > > Reported by Kernel Concurrency Sanitizer on: > > CPU: 1 PID: 11990 Comm: syz-executor.2 Not tainted 5.4.0-rc3+ #0 > > Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS > > Google 01/01/2011 > > ================================================================== > > I was trying to understand what is happening here, but fail to see how > this can happen. So it'd be good if somebody who knows this code can > explain. We are quite positive this is not a false positive, given the > addresses accessed match. Both of these accesses are into a buffer head; ie the data being accessed is stored in the page cache. Is it possible the page was reused for different data between these two accesses? > The two bits of code in question here are: > > static void fat16_ent_put(struct fat_entry *fatent, int new) > { > if (new == FAT_ENT_EOF) > new = EOF_FAT16; > > *fatent->u.ent16_p = cpu_to_le16(new); <<== data race here > mark_buffer_dirty_inode(fatent->bhs[0], fatent->fat_inode); > } > > int fat_search_long(struct inode *inode, const unsigned char *name, > int name_len, struct fat_slot_info *sinfo) > { > struct super_block *sb = inode->i_sb; > struct msdos_sb_info *sbi = MSDOS_SB(sb); > struct buffer_head *bh = NULL; > struct msdos_dir_entry *de; > unsigned char nr_slots; > wchar_t *unicode = NULL; > unsigned char bufname[FAT_MAX_SHORT_SIZE]; > loff_t cpos = 0; > int err, len; > > err = -ENOENT; > while (1) { > if (fat_get_entry(inode, &cpos, &bh, &de) == -1) > goto end_of_dir; > parse_record: > nr_slots = 0; > if (de->name[0] == DELETED_FLAG) > continue; > if (de->attr != ATTR_EXT && (de->attr & ATTR_VOLUME)) <<== data race here > continue; > if (de->attr != ATTR_EXT && IS_FREE(de->name)) > continue; > <snip> > } > > Thanks, > -- Marco ^ permalink raw reply [flat|nested] 4+ messages in thread
* Re: KCSAN: data-race in fat16_ent_put / fat_search_long 2019-11-05 15:25 ` Matthew Wilcox @ 2019-11-06 8:31 ` OGAWA Hirofumi 2019-11-06 11:26 ` Marco Elver 0 siblings, 1 reply; 4+ messages in thread From: OGAWA Hirofumi @ 2019-11-06 8:31 UTC (permalink / raw) To: Matthew Wilcox Cc: Marco Elver, syzbot, linux-kernel, syzkaller-bugs, linux-fsdevel, viro Matthew Wilcox <willy@infradead.org> writes: > On Tue, Nov 05, 2019 at 03:39:23PM +0100, Marco Elver wrote: >> On Tue, 05 Nov 2019, syzbot wrote: >> > ================================================================== >> > BUG: KCSAN: data-race in fat16_ent_put / fat_search_long >> > >> > write to 0xffff8880a209c96a of 2 bytes by task 11985 on cpu 0: >> > fat16_ent_put+0x5b/0x90 fs/fat/fatent.c:181 >> > fat_ent_write+0x6d/0xf0 fs/fat/fatent.c:415 >> > fat_chain_add+0x34e/0x400 fs/fat/misc.c:130 >> > fat_add_cluster+0x92/0xd0 fs/fat/inode.c:112 >> > __fat_get_block fs/fat/inode.c:154 [inline] >> > fat_get_block+0x3ae/0x4e0 fs/fat/inode.c:189 >> > __block_write_begin_int+0x2ea/0xf20 fs/buffer.c:1968 >> > __block_write_begin fs/buffer.c:2018 [inline] >> > block_write_begin+0x77/0x160 fs/buffer.c:2077 >> > cont_write_begin+0x3d6/0x670 fs/buffer.c:2426 >> > fat_write_begin+0x72/0xc0 fs/fat/inode.c:235 >> > pagecache_write_begin+0x6b/0x90 mm/filemap.c:3148 >> > cont_expand_zero fs/buffer.c:2353 [inline] >> > cont_write_begin+0x17a/0x670 fs/buffer.c:2416 >> > fat_write_begin+0x72/0xc0 fs/fat/inode.c:235 >> > pagecache_write_begin+0x6b/0x90 mm/filemap.c:3148 >> > generic_cont_expand_simple+0xb0/0x120 fs/buffer.c:2317 >> > >> > read to 0xffff8880a209c96b of 1 bytes by task 11990 on cpu 1: >> > fat_search_long+0x20a/0xc60 fs/fat/dir.c:484 >> > vfat_find+0xc1/0xd0 fs/fat/namei_vfat.c:698 >> > vfat_lookup+0x75/0x350 fs/fat/namei_vfat.c:712 >> > lookup_open fs/namei.c:3203 [inline] >> > do_last fs/namei.c:3314 [inline] >> > path_openat+0x15b6/0x36e0 fs/namei.c:3525 >> > do_filp_open+0x11e/0x1b0 fs/namei.c:3555 >> > do_sys_open+0x3b3/0x4f0 fs/open.c:1097 >> > __do_sys_open fs/open.c:1115 [inline] >> > __se_sys_open fs/open.c:1110 [inline] >> > __x64_sys_open+0x55/0x70 fs/open.c:1110 >> > do_syscall_64+0xcc/0x370 arch/x86/entry/common.c:290 >> > entry_SYSCALL_64_after_hwframe+0x44/0xa9 >> > >> > Reported by Kernel Concurrency Sanitizer on: >> > CPU: 1 PID: 11990 Comm: syz-executor.2 Not tainted 5.4.0-rc3+ #0 >> > Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS >> > Google 01/01/2011 >> > ================================================================== >> >> I was trying to understand what is happening here, but fail to see how >> this can happen. So it'd be good if somebody who knows this code can >> explain. We are quite positive this is not a false positive, given the >> addresses accessed match. > > Both of these accesses are into a buffer head; ie the data being accessed > is stored in the page cache. Is it possible the page was reused for > different data between these two accesses? No and yes. Reader side is directory buffer, writer side is FAT buffer. So FAT buffer never be reused as directory buffer. But the page cache itself can be freed and reused as different index. So if KCSAN can't detect the page cache recycle, it would be possible. Is there anyway to know "why KCSAN thought this as data race"? >> The two bits of code in question here are: >> >> static void fat16_ent_put(struct fat_entry *fatent, int new) >> { >> if (new == FAT_ENT_EOF) >> new = EOF_FAT16; >> >> *fatent->u.ent16_p = cpu_to_le16(new); <<== data race here >> mark_buffer_dirty_inode(fatent->bhs[0], fatent->fat_inode); >> } This is updating FAT entry (index for data cluster placement) on FAT buffer. >> int fat_search_long(struct inode *inode, const unsigned char *name, >> int name_len, struct fat_slot_info *sinfo) >> { >> struct super_block *sb = inode->i_sb; >> struct msdos_sb_info *sbi = MSDOS_SB(sb); >> struct buffer_head *bh = NULL; >> struct msdos_dir_entry *de; >> unsigned char nr_slots; >> wchar_t *unicode = NULL; >> unsigned char bufname[FAT_MAX_SHORT_SIZE]; >> loff_t cpos = 0; >> int err, len; >> >> err = -ENOENT; >> while (1) { >> if (fat_get_entry(inode, &cpos, &bh, &de) == -1) >> goto end_of_dir; >> parse_record: >> nr_slots = 0; >> if (de->name[0] == DELETED_FLAG) >> continue; >> if (de->attr != ATTR_EXT && (de->attr & ATTR_VOLUME)) <<== data race here Checking attribute on directory buffer. >> continue; >> if (de->attr != ATTR_EXT && IS_FREE(de->name)) >> continue; >> <snip> >> } >> >> Thanks, >> -- Marco -- OGAWA Hirofumi <hirofumi@mail.parknet.co.jp> ^ permalink raw reply [flat|nested] 4+ messages in thread
* Re: KCSAN: data-race in fat16_ent_put / fat_search_long 2019-11-06 8:31 ` OGAWA Hirofumi @ 2019-11-06 11:26 ` Marco Elver 0 siblings, 0 replies; 4+ messages in thread From: Marco Elver @ 2019-11-06 11:26 UTC (permalink / raw) To: OGAWA Hirofumi Cc: Matthew Wilcox, syzbot, LKML, syzkaller-bugs, linux-fsdevel, viro On Wed, 6 Nov 2019 at 09:31, OGAWA Hirofumi <hirofumi@mail.parknet.co.jp> wrote: > > Matthew Wilcox <willy@infradead.org> writes: > > > On Tue, Nov 05, 2019 at 03:39:23PM +0100, Marco Elver wrote: > >> On Tue, 05 Nov 2019, syzbot wrote: > >> > ================================================================== > >> > BUG: KCSAN: data-race in fat16_ent_put / fat_search_long > >> > > >> > write to 0xffff8880a209c96a of 2 bytes by task 11985 on cpu 0: > >> > fat16_ent_put+0x5b/0x90 fs/fat/fatent.c:181 > >> > fat_ent_write+0x6d/0xf0 fs/fat/fatent.c:415 > >> > fat_chain_add+0x34e/0x400 fs/fat/misc.c:130 > >> > fat_add_cluster+0x92/0xd0 fs/fat/inode.c:112 > >> > __fat_get_block fs/fat/inode.c:154 [inline] > >> > fat_get_block+0x3ae/0x4e0 fs/fat/inode.c:189 > >> > __block_write_begin_int+0x2ea/0xf20 fs/buffer.c:1968 > >> > __block_write_begin fs/buffer.c:2018 [inline] > >> > block_write_begin+0x77/0x160 fs/buffer.c:2077 > >> > cont_write_begin+0x3d6/0x670 fs/buffer.c:2426 > >> > fat_write_begin+0x72/0xc0 fs/fat/inode.c:235 > >> > pagecache_write_begin+0x6b/0x90 mm/filemap.c:3148 > >> > cont_expand_zero fs/buffer.c:2353 [inline] > >> > cont_write_begin+0x17a/0x670 fs/buffer.c:2416 > >> > fat_write_begin+0x72/0xc0 fs/fat/inode.c:235 > >> > pagecache_write_begin+0x6b/0x90 mm/filemap.c:3148 > >> > generic_cont_expand_simple+0xb0/0x120 fs/buffer.c:2317 > >> > > >> > read to 0xffff8880a209c96b of 1 bytes by task 11990 on cpu 1: > >> > fat_search_long+0x20a/0xc60 fs/fat/dir.c:484 > >> > vfat_find+0xc1/0xd0 fs/fat/namei_vfat.c:698 > >> > vfat_lookup+0x75/0x350 fs/fat/namei_vfat.c:712 > >> > lookup_open fs/namei.c:3203 [inline] > >> > do_last fs/namei.c:3314 [inline] > >> > path_openat+0x15b6/0x36e0 fs/namei.c:3525 > >> > do_filp_open+0x11e/0x1b0 fs/namei.c:3555 > >> > do_sys_open+0x3b3/0x4f0 fs/open.c:1097 > >> > __do_sys_open fs/open.c:1115 [inline] > >> > __se_sys_open fs/open.c:1110 [inline] > >> > __x64_sys_open+0x55/0x70 fs/open.c:1110 > >> > do_syscall_64+0xcc/0x370 arch/x86/entry/common.c:290 > >> > entry_SYSCALL_64_after_hwframe+0x44/0xa9 > >> > > >> > Reported by Kernel Concurrency Sanitizer on: > >> > CPU: 1 PID: 11990 Comm: syz-executor.2 Not tainted 5.4.0-rc3+ #0 > >> > Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS > >> > Google 01/01/2011 > >> > ================================================================== > >> > >> I was trying to understand what is happening here, but fail to see how > >> this can happen. So it'd be good if somebody who knows this code can > >> explain. We are quite positive this is not a false positive, given the > >> addresses accessed match. > > > > Both of these accesses are into a buffer head; ie the data being accessed > > is stored in the page cache. Is it possible the page was reused for > > different data between these two accesses? > > No and yes. Reader side is directory buffer, writer side is FAT buffer. > So FAT buffer never be reused as directory buffer. But the page cache > itself can be freed and reused as different index. So if KCSAN can't > detect the page cache recycle, it would be possible. > > Is there anyway to know "why KCSAN thought this as data race"? KCSAN set up a watchpoint on the plain read, simply stalling that thread for a few microsec. While stalling, a concurrent plain write occurred which matches the watchpoint the reader set up. Whenever KCSAN detects a data race, the 2 operations *must* actually be happening in parallel at the time. I will try to reproduce this somehow. > >> The two bits of code in question here are: > >> > >> static void fat16_ent_put(struct fat_entry *fatent, int new) > >> { > >> if (new == FAT_ENT_EOF) > >> new = EOF_FAT16; > >> > >> *fatent->u.ent16_p = cpu_to_le16(new); <<== data race here > >> mark_buffer_dirty_inode(fatent->bhs[0], fatent->fat_inode); > >> } > > This is updating FAT entry (index for data cluster placement) on FAT buffer. > > >> int fat_search_long(struct inode *inode, const unsigned char *name, > >> int name_len, struct fat_slot_info *sinfo) > >> { > >> struct super_block *sb = inode->i_sb; > >> struct msdos_sb_info *sbi = MSDOS_SB(sb); > >> struct buffer_head *bh = NULL; > >> struct msdos_dir_entry *de; > >> unsigned char nr_slots; > >> wchar_t *unicode = NULL; > >> unsigned char bufname[FAT_MAX_SHORT_SIZE]; > >> loff_t cpos = 0; > >> int err, len; > >> > >> err = -ENOENT; > >> while (1) { > >> if (fat_get_entry(inode, &cpos, &bh, &de) == -1) > >> goto end_of_dir; > >> parse_record: > >> nr_slots = 0; > >> if (de->name[0] == DELETED_FLAG) > >> continue; > >> if (de->attr != ATTR_EXT && (de->attr & ATTR_VOLUME)) <<== data race here > > Checking attribute on directory buffer. > > >> continue; > >> if (de->attr != ATTR_EXT && IS_FREE(de->name)) > >> continue; > >> <snip> > >> } > >> > >> Thanks, > >> -- Marco > > -- > OGAWA Hirofumi <hirofumi@mail.parknet.co.jp> ^ permalink raw reply [flat|nested] 4+ messages in thread
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2019-11-05 14:39 ` KCSAN: data-race in fat16_ent_put / fat_search_long Marco Elver
2019-11-05 15:25 ` Matthew Wilcox
2019-11-06 8:31 ` OGAWA Hirofumi
2019-11-06 11:26 ` Marco Elver
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