[PATCH 3.4 90/92] pipe: Fix buffer offset after partially failed read

lizf
2016-04-18T10:52:52Z

From: Ben Hutchings <ben@decadent.org.uk>

3.4.112-rc1 review patch. If anyone has any objections, please let me know.

------------------

Quoting the RHEL advisory:

> 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 <ben@decadent.org.uk>
Cc: Jeffrey Vander Stoep <jeffv@google.com>
Signed-off-by: Zefan Li <lizefan@huawei.com>
---
fs/pipe.c | 5 ++++-
1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/fs/pipe.c b/fs/pipe.c
index abfb935..6049235 100644
--- a/fs/pipe.c
+++ b/fs/pipe.c
@@ -390,6 +390,7 @@ pipe_read(struct kiocb *iocb, const struct iovec *_iov,
void *addr;
size_t chars = buf->len, remaining;
int error, atomic;
+ int offset;

if (chars > total_len)
chars = total_len;
@@ -403,9 +404,10 @@ pipe_read(struct kiocb *iocb, const struct iovec *_iov,

atomic = !iov_fault_in_pages_write(iov, chars);
remaining = chars;
+ 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)) {
@@ -421,6 +423,7 @@ redo:
break;
}
ret += chars;
+ buf->offset += chars;
buf->len -= chars;

/* Was it a packet buffer? Clean up and exit */
--
1.9.1