[PATCH 4.14 157/183] nfsd: fix memory corruption caused by readdir

From: Greg Kroah-Hartman
Date: Fri Mar 22 2019 - 07:55:18 EST

4.14-stable review patch. If anyone has any objections, please let me know.


From: NeilBrown <neilb@xxxxxxxx>

commit b602345da6cbb135ba68cf042df8ec9a73da7981 upstream.

If the result of an NFSv3 readdir{,plus} request results in the
"offset" on one entry having to be split across 2 pages, and is sized
so that the next directory entry doesn't fit in the requested size,
then memory corruption can happen.

When encode_entry() is called after encoding the last entry that fits,
it notices that ->offset and ->offset1 are set, and so stores the
offset value in the two pages as required. It clears ->offset1 but
*does not* clear ->offset.

Normally this omission doesn't matter as encode_entry_baggage() will
be called, and will set ->offset to a suitable value (not on a page
But in the case where cd->buflen < elen and nfserr_toosmall is
returned, ->offset is not reset.

This means that nfsd3proc_readdirplus will see ->offset with a value 4
bytes before the end of a page, and ->offset1 set to NULL.
It will try to write 8bytes to ->offset.
If we are lucky, the next page will be read-only, and the system will
BUG: unable to handle kernel paging request at...

If we are unlucky, some innocent page will have the first 4 bytes

nfsd3proc_readdir() doesn't even check for ->offset1, it just blindly
writes 8 bytes to the offset wherever it is.

Fix this by clearing ->offset after it is used, and copying the
->offset handling code from nfsd3_proc_readdirplus into

(Note that the commit hash in the Fixes tag is from the 'history'
tree - this bug predates git).

Fixes: 0b1d57cf7654 ("[PATCH] kNFSd: Fix nfs3 dentry encoding")
Fixes-URL: https://git.kernel.org/pub/scm/linux/kernel/git/history/history.git/commit/?id=0b1d57cf7654
Cc: stable@xxxxxxxxxxxxxxx (v2.6.12+)
Signed-off-by: NeilBrown <neilb@xxxxxxxx>
Signed-off-by: J. Bruce Fields <bfields@xxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

fs/nfsd/nfs3proc.c | 16 ++++++++++++++--
fs/nfsd/nfs3xdr.c | 1 +
2 files changed, 15 insertions(+), 2 deletions(-)

--- a/fs/nfsd/nfs3proc.c
+++ b/fs/nfsd/nfs3proc.c
@@ -446,8 +446,19 @@ nfsd3_proc_readdir(struct svc_rqst *rqst
&resp->common, nfs3svc_encode_entry);
memcpy(resp->verf, argp->verf, 8);
resp->count = resp->buffer - argp->buffer;
- if (resp->offset)
- xdr_encode_hyper(resp->offset, argp->cookie);
+ if (resp->offset) {
+ loff_t offset = argp->cookie;
+ if (unlikely(resp->offset1)) {
+ /* we ended up with offset on a page boundary */
+ *resp->offset = htonl(offset >> 32);
+ *resp->offset1 = htonl(offset & 0xffffffff);
+ resp->offset1 = NULL;
+ } else {
+ xdr_encode_hyper(resp->offset, offset);
+ }
+ resp->offset = NULL;
+ }

@@ -516,6 +527,7 @@ nfsd3_proc_readdirplus(struct svc_rqst *
} else {
xdr_encode_hyper(resp->offset, offset);
+ resp->offset = NULL;

--- a/fs/nfsd/nfs3xdr.c
+++ b/fs/nfsd/nfs3xdr.c
@@ -922,6 +922,7 @@ encode_entry(struct readdir_cd *ccd, con
} else {
xdr_encode_hyper(cd->offset, offset64);
+ cd->offset = NULL;