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

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

[Eric Biggers]

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

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

[Kees Cook]

On Sun, Jan 7, 2018 at 9:35 PM, Eric Biggers <ebiggers3@gmail.com> wrote:
> 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.

Should this be controlled per-namespace instead of via init-ns caps?

-Kees

>
> 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
>



-- 
Kees Cook
Pixel Security

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

[Eric Biggers]

On Tue, Jan 09, 2018 at 02:23:32PM -0800, Kees Cook wrote:
> On Sun, Jan 7, 2018 at 9:35 PM, Eric Biggers <ebiggers3@gmail.com> wrote:
> > 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.
> 
> Should this be controlled per-namespace instead of via init-ns caps?
> 

I don't think so.  Users shouldn't be able to bypass the limits by creating a
user namespace.

Eric