On Wed, 15 Jul 2020 at 18:33, Eric Biggers <ebiggers@xxxxxxxxxx> wrote: > > [+Cc linux-fsdevel] > > On Wed, Jul 15, 2020 at 05:29:12PM +0200, 'Marco Elver' via syzkaller-bugs wrote: > > On Wed, Jul 15, 2020 at 08:16AM -0700, syzbot wrote: > > > Hello, > > > > > > syzbot found the following issue on: > > > > > > HEAD commit: e9919e11 Merge branch 'for-linus' of git://git.kernel.org/.. > > > git tree: upstream > > > console output: https://syzkaller.appspot.com/x/log.txt?x=1217a83b100000 > > > kernel config: https://syzkaller.appspot.com/x/.config?x=570eb530a65cd98e > > > dashboard link: https://syzkaller.appspot.com/bug?extid=0f1e470df6a4316e0a11 > > > compiler: clang version 11.0.0 (https://github.com/llvm/llvm-project.git ca2dcbd030eadbf0aa9b660efe864ff08af6e18b) > > > > > > Unfortunately, I don't have any reproducer for this issue yet. > > > > > > IMPORTANT: if you fix the issue, please add the following tag to the commit: > > > Reported-by: syzbot+0f1e470df6a4316e0a11@xxxxxxxxxxxxxxxxxxxxxxxxx > > > > > > ================================================================== > > > BUG: KCSAN: data-race in generic_file_buffered_read / generic_file_buffered_read > > > > Our guess is that this is either misuse of an API from userspace, or a > > bug. Can someone clarify? > > > > Below are the snippets of code around these accesses. > > Concurrent reads on the same file descriptor are allowed. Not with sys_read(), > as that implicitly uses the file position. But it's allowed with sys_pread(), > and also with sys_sendfile() which is the case syzbot is reporting here. > > > > > > write to 0xffff8880968747b0 of 8 bytes by task 6336 on cpu 0: > > > generic_file_buffered_read+0x18be/0x19e0 mm/filemap.c:2246 > > > > ... > > would_block: > > error = -EAGAIN; > > out: > > ra->prev_pos = prev_index; > > ra->prev_pos <<= PAGE_SHIFT; > > 2246) ra->prev_pos |= prev_offset; > > > > *ppos = ((loff_t)index << PAGE_SHIFT) + offset; > > file_accessed(filp); > > return written ? written : error; > > } > > EXPORT_SYMBOL_GPL(generic_file_buffered_read); > > ... > > Well, it's a data race. Each open file descriptor has just one readahead state > (struct file_ra_state), and concurrent reads of the same file descriptor > use/change that readahead state without any locking. > > Presumably this has traditionally been considered okay, since readahead is > "only" for performance and doesn't affect correctness. And for performance > reasons, we want to avoid locking during file reads. > > So we may just need to annotate all access to file_ra_state with > READ_ONCE() and WRITE_ONCE()... The thing that stood out here are the multiple accesses both on the reader and writer side. If it was only 1 access, where the race is expected, a simple READ/WRITE_ONCE might have been OK. But here, we actually have several writes to the same variable 'prev_pos'. The reader is also doing several reads to the same variable. Maybe we got lucky because the compiler just turns it into 1 load, keeps it in a register and does the various modifications, and then 1 store to write back. Similar on the reader side, we may have gotten lucky in that the compiler just does 1 actual load. If that behaviour is safe, it needs to be made explicit to make it impossible for the compiler to generate anything else. > > > generic_file_read_iter+0x7d/0x3e0 mm/filemap.c:2326 > > > ext4_file_read_iter+0x2d6/0x420 fs/ext4/file.c:74 > > > call_read_iter include/linux/fs.h:1902 [inline] > > > generic_file_splice_read+0x22a/0x310 fs/splice.c:312 > > > do_splice_to fs/splice.c:870 [inline] > > > splice_direct_to_actor+0x2a8/0x660 fs/splice.c:950 > > > do_splice_direct+0xf2/0x170 fs/splice.c:1059 > > > do_sendfile+0x562/0xb10 fs/read_write.c:1540 > > > __do_sys_sendfile64 fs/read_write.c:1601 [inline] > > > __se_sys_sendfile64 fs/read_write.c:1587 [inline] > > > __x64_sys_sendfile64+0xf2/0x130 fs/read_write.c:1587 > > > do_syscall_64+0x51/0xb0 arch/x86/entry/common.c:384 > > > entry_SYSCALL_64_after_hwframe+0x44/0xa9 > > > > > > read to 0xffff8880968747b0 of 8 bytes by task 6334 on cpu 1: > > > generic_file_buffered_read+0x11e/0x19e0 mm/filemap.c:2011 > > > > ... > > index = *ppos >> PAGE_SHIFT; > > prev_index = ra->prev_pos >> PAGE_SHIFT; > > 2011) prev_offset = ra->prev_pos & (PAGE_SIZE-1); > > last_index = (*ppos + iter->count + PAGE_SIZE-1) >> PAGE_SHIFT; > > offset = *ppos & ~PAGE_MASK; > > ... > > > > > generic_file_read_iter+0x7d/0x3e0 mm/filemap.c:2326 > > > ext4_file_read_iter+0x2d6/0x420 fs/ext4/file.c:74 > > > call_read_iter include/linux/fs.h:1902 [inline] > > > generic_file_splice_read+0x22a/0x310 fs/splice.c:312 > > > do_splice_to fs/splice.c:870 [inline] > > > splice_direct_to_actor+0x2a8/0x660 fs/splice.c:950 > > > do_splice_direct+0xf2/0x170 fs/splice.c:1059 > > > do_sendfile+0x562/0xb10 fs/read_write.c:1540 > > > __do_sys_sendfile64 fs/read_write.c:1601 [inline] > > > __se_sys_sendfile64 fs/read_write.c:1587 [inline] > > > __x64_sys_sendfile64+0xf2/0x130 fs/read_write.c:1587 > > > do_syscall_64+0x51/0xb0 arch/x86/entry/common.c:384 > > > entry_SYSCALL_64_after_hwframe+0x44/0xa9 > > > > > > Reported by Kernel Concurrency Sanitizer on: > > > CPU: 1 PID: 6334 Comm: syz-executor.0 Not tainted 5.8.0-rc5-syzkaller #0 > > > Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011 > > > ==================================================================
