[PATCH 3/7] pipe: actually allow root to exceed the pipe buffer limits

Eric Biggers
2018-01-08T05:38:57Z

From: Eric Biggers <ebiggers@google.com>

pipe-user-pages-hard and pipe-user-pages-soft are only supposed to apply
to unprivileged users, as documented in both Documentation/sysctl/fs.txt
and the pipe(7) man page.

However, the capabilities are actually only checked when increasing a
pipe's size using F_SETPIPE_SZ, not when creating a new pipe.
Therefore, if pipe-user-pages-hard has been set, the root user can run
into it and be unable to create pipes. Similarly, if
pipe-user-pages-soft has been set, the root user can run into it and
have their pipes limited to 1 page each.

Fix this by allowing the privileged override in both cases.

Fixes: 759c01142a5d ("pipe: limit the per-user amount of pages allocated in pipes")
Cc: stable@vger.kernel.org
Signed-off-by: Eric Biggers <ebiggers@google.com>
---
fs/pipe.c | 11 ++++++++---
1 file changed, 8 insertions(+), 3 deletions(-)

diff --git a/fs/pipe.c b/fs/pipe.c
index d0dec5e7ef33..847ecc388820 100644
--- a/fs/pipe.c
+++ b/fs/pipe.c
@@ -613,6 +613,11 @@ static bool too_many_pipe_buffers_hard(unsigned long user_bufs)
return pipe_user_pages_hard && user_bufs >= pipe_user_pages_hard;
}

+static bool is_unprivileged_user(void)
+{
+ return !capable(CAP_SYS_RESOURCE) && !capable(CAP_SYS_ADMIN);
+}
+
struct pipe_inode_info *alloc_pipe_info(void)
{
struct pipe_inode_info *pipe;
@@ -629,12 +634,12 @@ struct pipe_inode_info *alloc_pipe_info(void)

user_bufs = account_pipe_buffers(user, 0, pipe_bufs);

- if (too_many_pipe_buffers_soft(user_bufs)) {
+ if (too_many_pipe_buffers_soft(user_bufs) && is_unprivileged_user()) {
user_bufs = account_pipe_buffers(user, pipe_bufs, 1);
pipe_bufs = 1;
}

- if (too_many_pipe_buffers_hard(user_bufs))
+ if (too_many_pipe_buffers_hard(user_bufs) && is_unprivileged_user())
goto out_revert_acct;

pipe->bufs = kcalloc(pipe_bufs, sizeof(struct pipe_buffer),
@@ -1065,7 +1070,7 @@ static long pipe_set_size(struct pipe_inode_info *pipe, unsigned long arg)
if (nr_pages > pipe->buffers &&
(too_many_pipe_buffers_hard(user_bufs) ||
too_many_pipe_buffers_soft(user_bufs)) &&
- !capable(CAP_SYS_RESOURCE) && !capable(CAP_SYS_ADMIN)) {
+ is_unprivileged_user()) {
ret = -EPERM;
goto out_revert_acct;
}
--
2.15.1