On Tue, 27 Nov 2012 16:41:28 +0530
Suresh Jayaraman <sjayaraman@xxxxxxxx> wrote:
> On 11/27/2012 04:34 PM, Jeff Layton wrote:
> > On Tue, 27 Nov 2012 12:35:37 +0530
> > Suresh Jayaraman <sjayaraman@xxxxxxxx> wrote:
> >
> >> On 11/26/2012 09:28 PM, Jeff Layton wrote:
> >>> Commit eddb079deb4 created a regression in the writepages codepath.
> >>> Previously, whenever it needed to check the size of the file, it did so
> >>> by consulting the inode->i_size field directly. With that patch, the
> >>> i_size was fetched once on entry into the writepages code and that value
> >>> was used henceforth.
> >>>
> >>> If the file is changing size though (for instance, if someone is writing
> >>> to it or has truncated it), then that value is likely to be wrong. This
> >>> can lead to data corruption. Pages past the EOF at the time that the
> >>> writepages call was issued may be silently dropped and ignored because
> >>> cifs_writepages wrongly assumes that the file must have been truncated
> >>> in the interim.
> >>>
> >>> Fix cifs_writepages to properly fetch the size from the inode->i_size
> >>> field instead to properly account for this possibility.
> >>>
> >>> Reported-and-Tested-by: Maxim Britov <ungifted01@xxxxxxxxx>
> >>> Signed-off-by: Jeff Layton <jlayton@xxxxxxxxxx>
> >>> ---
> >>> fs/cifs/file.c | 6 +++---
> >>> 1 file changed, 3 insertions(+), 3 deletions(-)
> >>>
> >>> diff --git a/fs/cifs/file.c b/fs/cifs/file.c
> >>> index edb25b4..70b6f4c 100644
> >>> --- a/fs/cifs/file.c
> >>> +++ b/fs/cifs/file.c
> >>> @@ -1794,7 +1794,6 @@ static int cifs_writepages(struct address_space *mapping,
> >>> struct TCP_Server_Info *server;
> >>> struct page *page;
> >>> int rc = 0;
> >>> - loff_t isize = i_size_read(mapping->host);
> >>>
> >>> /*
> >>> * If wsize is smaller than the page cache size, default to writing
> >>> @@ -1899,7 +1898,7 @@ retry:
> >>> */
> >>> set_page_writeback(page);
> >>>
> >>> - if (page_offset(page) >= isize) {
> >>> + if (page_offset(page) >= i_size_read(mapping->host)) {
> >>> done = true;
> >>> unlock_page(page);
> >>> end_page_writeback(page);
> >>> @@ -1932,7 +1931,8 @@ retry:
> >>> wdata->offset = page_offset(wdata->pages[0]);
> >>> wdata->pagesz = PAGE_CACHE_SIZE;
> >>> wdata->tailsz =
> >>> - min(isize - page_offset(wdata->pages[nr_pages - 1]),
> >>> + min(i_size_read(mapping->host) -
> >>> + page_offset(wdata->pages[nr_pages - 1]),
> >>> (loff_t)PAGE_CACHE_SIZE);
> >>
> >> Good catch. Looks correct to me. Wondering whether we would need a
> >> similar fix in cifs_write_begin() where we get the i_size and then
> >> immediately do check whether the page lies beyond EOF?
> >>
> >>
> >
> > Thanks, I should also mention that this was reported here:
> >
> > https://bugzilla.kernel.org/show_bug.cgi?id=50991
> >
> > Regarding cifs_write_begin, I don't think so. The code does not sleep
> > between the point where the i_size is fetched and then later used. If
> > there is a race window there, it's very small.
> >
>
> Yeah, I too was thinking it is too small a race window but thought
> anyway I'll ask.
>
> Steve, I think this has to be pushed to Linus asap before the merge
> window begins to ensure that 3.7 includes this fix.
>
>
Agreed -- this shouldn't wait...
--
Jeff Layton <jlayton@xxxxxxxxxx>
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