At 19:19 08/09/10, Nick Piggin wrote: >On Wed, Sep 10, 2008 at 05:47:00PM +0900, Hisashi Hifumi wrote: >> >> At 13:52 08/09/10, Nick Piggin wrote: >> > >> >Patch 8ab22b9a, "vfs: pagecache usage optimization for pagesize!=blocksize", >> >introduces a data race that might cause uninitialized data to be exposed to >> >userland. The race is conceptually the same as the one fixed for page >> >uptodateness, fixed by 0ed361de. >> > >> >The problem is that a buffer_head flags will be set uptodate after the >> >stores to bring its pagecache data uptodate[*]. This patch introduces a >> >possibility to read that pagecache data if the buffer_head flag has been >> >found uptodate. The problem is there are no barriers or locks ordering >> >the store/store vs the load/load. >> > >> >To illustrate: >> > CPU0: write(2) (1024 bytes) CPU1: read(2) (1024 bytes) >> > 1. allocate new pagecache page A. locate page, not fully uptodate >> > 2. copy_from_user to part of page B. partially uptodate? load bh flags >> > 3. mark that buffer uptodate C. if yes, then copy_to_user >> > >> >So if the store 3 is allowed to execute before the store 2, and/or the >> >load in C is allowed to execute before the load in B, then we can wind >> >up loading !uptodate data. >> > >> >> > >> >One way to solve this is to add barriers to the buffer head operations >> >similarly to the fix for the page issue. The problem is that, unlike the >> >page race, we don't actually *need* to do that if we decide not to support >> >this functionality. The barriers are quite heavyweight on some >> >architectures, and we haven't seen really compelling numbers in favour of >> >this patch yet (a best-case microbenchmark showed some improvement of >> >course, but with memory barriers we could also produce a worst-case bench >> >that shows some slowdown on many architectures). >> >> I think that adding wmb/rmb to all buffer_uptodate/set_buffer_uptodate is heavy >> on some architectures using BUFFER_FNS macros, but it can be possible >> to mitigate performance slowdown by minimizing memory barrier utilization. >> The patch "vfs: pagecache usage optimization for pagesize!=blocksize" is now >> just for ext2/3/4, so is it not sufficient to solve the above >uninitialized data >> exposure problem that adding one rmb to block_is_partially_uptodate() >> and wmb to __block_commit_write() ? > >I guess it could be... if you have audited all those filesystems to ensure >they don't set the buffer uptodate via any other paths. > >But still, forcing a wmb for everyone in the block path is... not so nice. >As I said, I think the _best_ way to solve the problem is to ensure the >buffer is only brought uptodate under the page lock, which will then give >you serialisation against block_is_partially_uptodate (which is called with >the page locked). If you are *sure* this is the case for ext2/3/4, then there >should actually be no memory ordering problem in practice. You will have to >document the API to say that users of it must obey that rule. > I again investigated write() path on ext2/3/4. On these filesystems set_buffer_uptodate is done through __block_prepare_write or __block_commit_write(), and is inside lock_page. buffer_uptodate test in block_is_partially_uptodate and set_buffer_uptodate in __block_prepare_write or __block_commit_write() are done inside lock_page, so I think these bitops is serialized and there is no memory ordering problem regarding buffer_uptodate/set_buffer_uptodate as far as ext2/3/4 is concerned. -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
