pipe: Fix buffer offset after partially failed read

It was found that the fix for CVE-2015-1805 incorrectly kept buffer offset and buffer length in sync on a failed atomic read, potentially resulting in a pipe buffer state corruption. A local, unprivileged user could use this flaw to crash the system or leak kernel memory to user space. (CVE-2016-0774, Moderate)

Quoting the RHEL advisory:

Quoted 5 lines

It was found that the fix for CVE-2015-1805 incorrectly kept buffer offset and buffer length in sync on a failed atomic read, potentially resulting in a pipe buffer state corruption. A local, unprivileged user could use this flaw to crash the system or leak kernel memory to user space. (CVE-2016-0774, Moderate)

The same flawed fix was applied to stable branches from 2.6.32.y to 3.14.y inclusive, and I was able to reproduce the issue on 3.2.y. We need to give pipe_iov_copy_to_user() a separate offset variable and only update the buffer offset if it succeeds.

Quoting the RHEL advisory:

Quoted 5 lines

It was found that the fix for CVE-2015-1805 incorrectly kept buffer offset and buffer length in sync on a failed atomic read, potentially resulting in a pipe buffer state corruption. A local, unprivileged user could use this flaw to crash the system or leak kernel memory to user space. (CVE-2016-0774, Moderate)

The same flawed fix was applied to stable branches from 2.6.32.y to 3.14.y inclusive, and I was able to reproduce the issue on 3.2.y. We need to give pipe_iov_copy_to_user() a separate offset variable and only update the buffer offset if it succeeds.

References: https://rhn.redhat.com/errata/RHSA-2016-0103.html

Signed-off-by Ben Hutchings <[email protected]>

--- a/fs/pipe.c +++ b/fs/pipe.c @@ -395,6 +395,7 @@ pipe_read(struct kiocb *iocb, const stru void *addr; size_t chars =
buf->len, remaining; int error, atomic; + int offset;

if (chars > total_len) chars = total_len; @@ -408,9 +409,10 @@ pipe_read(struct kiocb *iocb, const stru

atomic = !iov_fault_in_pages_write(iov, chars); remaining = chars;

(quoted diff) +1 -0

+			offset = buf->offset; redo: addr = ops->map(pipe, buf,

atomic); - error = pipe_iov_copy_to_user(iov, addr,
&buf->offset, + error = pipe_iov_copy_to_user(iov, addr,
&offset, &remaining, atomic); ops->unmap(pipe, buf, addr); if (unlikely(error)) { @@ -426,6 +428,7 @@ redo: break; } ret +=
chars; + buf->offset += chars; buf->len -= chars;

/* Was it a packet buffer? Clean up and exit */