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Michael Kerrisk (man-pages) authored
The limit checking in alloc_pipe_info() (used by pipe(2) and when opening a FIFO) has the following problems: (1) When checking capacity required for the new pipe, the checks against the limit in /proc/sys/fs/pipe-user-pages-{soft,hard} are made against existing consumption, and exclude the memory required for the new pipe capacity. As a consequence: (1) the memory allocation throttling provided by the soft limit does not kick in quite as early as it should, and (2) the user can overrun the hard limit. (2) As currently implemented, accounting and checking against the limits is done as follows: (a) Test whether the user has exceeded the limit. (b) Make new pipe buffer allocation. (c) Account new allocation against the limits. This is racey. Multiple processes may pass point (a) simultaneously, and then allocate pipe buffers that are accounted for only in step (c). The race means that the user's pipe buffer allocation could be pushed over the limit (by an arbitrary amount, depending on how unlucky we were in the race). [Thanks to Vegard Nossum for spotting this point, which I had missed.] This patch addresses the above problems as follows: * Alter the checks against limits to include the memory required for the new pipe. * Re-order the accounting step so that it precedes the buffer allocation. If the accounting step determines that a limit has been reached, revert the accounting and cause the operation to fail. Link: http://lkml.kernel.org/r/8ff3e9f9-23f6-510c-644f-8e70cd1c0bd9@gmail.comSigned-off-by:
Michael Kerrisk <mtk.manpages@gmail.com> Reviewed-by:
Vegard Nossum <vegard.nossum@oracle.com> Cc: Willy Tarreau <w@1wt.eu> Cc: <socketpair@gmail.com> Cc: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp> Cc: Jens Axboe <axboe@fb.com> Cc: Al Viro <viro@zeniv.linux.org.uk> Signed-off-by:
Andrew Morton <akpm@linux-foundation.org> Signed-off-by:
Linus Torvalds <torvalds@linux-foundation.org>
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