On Wed, Dec 09, 2020 at 08:14:15PM +0000, Matthew Wilcox wrote: > On Wed, Dec 09, 2020 at 11:47:56AM -0800, Dan Williams wrote: > > On Tue, Dec 8, 2020 at 8:03 PM Matthew Wilcox <wi...@infradead.org> wrote: > > > On Tue, Dec 08, 2020 at 06:22:50PM -0800, Ira Weiny wrote: > > > > Therefore, I tend to agree with Dan that if anything is to be done it > > > > should be > > > > a WARN_ON() which is only going to throw an error that something has > > > > probably > > > > been wrong all along and should be fixed but continue running as before. > > > > > > Silent data corruption is for ever. Are you absolutely sure nobody has > > > done: > > > > > > page = alloc_pages(GFP_HIGHUSER_MOVABLE, 3); > > > memcpy_to_page(page, PAGE_SIZE * 2, p, PAGE_SIZE * 2); > > > > > > because that will work fine if the pages come from ZONE_NORMAL and fail > > > miserably if they came from ZONE_HIGHMEM. > > > > ...and violently regress with the BUG_ON. > > ... which is what we want, no? > > > The question to me is: which is more likely that any bad usages have > > been covered up by being limited to ZONE_NORMAL / 64-bit only, or that > > silent data corruption has been occurring with no ill effects? > > I wouldn't be at all surprised to learn that there is silent data > corruption on 32-bit systems with HIGHMEM. Would you? How much testing > do you do on 32-bit HIGHMEM systems? > > Actually, I wouldn't be at all surprised if we can hit this problem today. > Look at this: > > size_t _copy_from_iter(void *addr, size_t bytes, struct iov_iter *i) > { > char *to = addr; > if (unlikely(iov_iter_is_pipe(i))) { > WARN_ON(1); > return 0; > } > if (iter_is_iovec(i)) > might_fault(); > iterate_and_advance(i, bytes, v, > copyin((to += v.iov_len) - v.iov_len, v.iov_base, v.iov_len), > memcpy_from_page((to += v.bv_len) - v.bv_len, v.bv_page, > v.bv_offset, v.bv_len), > memcpy((to += v.iov_len) - v.iov_len, v.iov_base, v.iov_len) > ) > > return bytes; > } > EXPORT_SYMBOL(_copy_from_iter); > > There's a lot of macrology in there, so for those following along who > aren't familiar with the iov_iter code, if the iter is operating on a > bvec, then iterate_and_advance() will call memcpy_from_page(), passing > it the bv_page, bv_offset and bv_len stored in the bvec. Since 2019, > Linux has supported multipage bvecs (commit 07173c3ec276). So bv_len > absolutely *can* be > PAGE_SIZE. > > Does this ever happen in practice? I have no idea; I don't know whether > any multipage BIOs are currently handed to copy_from_iter(). But I > have no confidence in your audit if you didn't catch this one.
Ah... This call site has been there since 2014 and is not a new caller I have been 'auditing'.[1] > > > > > FWIW I think this is a 'bad BUG_ON' use because we are "checking > > > > something that > > > > we know we might be getting wrong".[1] And because, "BUG() is only > > > > good for > > > > something that never happens and that we really have no other option > > > > for".[2] > > > > > > BUG() is our only option here. Both limiting how much we copy or > > > copying the requested amount result in data corruption or leaking > > > information to a process that isn't supposed to see it. > > > > At a minimum I think this should be debated in a follow on patch to > > add assertion checking where there was none before. There is no > > evidence of a page being overrun in the audit Ira performed. > > If we put in into a separate patch, someone will suggest backing out the > patch which tells us that there's a problem. You know, like this guy ... > https://lore.kernel.org/linux-mm/CAPcyv4jNVroYmirzKw_=cseixoeacdl3m1wc4xjd_tfv3h+...@mail.gmail.com/ I'm not following this. Regardless I've already added the BUG_ON's. Ira [1] commit 0dbca9a4b5d69a7e4b8c1d55b98312fcd9aafcf7 Author: Al Viro <v...@zeniv.linux.org.uk> Date: Thu Nov 27 14:26:43 2014 -0500 iov_iter.c: convert copy_from_iter() to iterate_and_advance Signed-off-by: Al Viro <v...@zeniv.linux.org.uk>