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.
--
Suresh Jayaraman
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