[PATCH] NFSD: check for negative "len" values in nfssvc_decode_writeargs()

[PATCH] NFSD: check for negative "len" values in nfssvc_decode_writeargs()

[Dan Carpenter]

This code checks the upper bound of "len" but it needs to check for
negative values as well.

Fixes: a51b5b737a0b ("NFSD: Update the NFSv2 WRITE argument decoder to use struct xdr_stream")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
---
From static analysis and I am not sure of the implications of this bug.

 fs/nfsd/nfsxdr.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/nfsd/nfsxdr.c b/fs/nfsd/nfsxdr.c
index aba8520b4b8b..a9f80e30320e 100644
--- a/fs/nfsd/nfsxdr.c
+++ b/fs/nfsd/nfsxdr.c
@@ -336,7 +336,7 @@ nfssvc_decode_writeargs(struct svc_rqst *rqstp, struct xdr_stream *xdr)
 	/* opaque data */
 	if (xdr_stream_decode_u32(xdr, &args->len) < 0)
 		return false;
-	if (args->len > NFSSVC_MAXBLKSIZE_V2)
+	if (args->len < 0 || args->len > NFSSVC_MAXBLKSIZE_V2)
 		return false;
 	if (!xdr_stream_subsegment(xdr, &args->payload, args->len))
 		return false;
-- 
2.20.1

Re: [PATCH] NFSD: check for negative "len" values in nfssvc_decode_writeargs()

[Chuck Lever III]

Hi Dan-

> On Mar 14, 2022, at 10:06 AM, Dan Carpenter <dan.carpenter@oracle.com> wrote:
> 
> This code checks the upper bound of "len" but it needs to check for
> negative values as well.

It doesn't check because nfsd3_writeargs::len is a __u32,
and the NFSv2 code here was copied from that assuming that
nfsd_writeargs::len had the same signage. This is because...

https://datatracker.ietf.org/doc/html/rfc1832#section-3.13 says
that the count field in a variable-length array is supposed to
be unsigned.

Thus IMO nfsd_writeargs::len should be changed to __u32
instead of adding the extra negativity check.

If you resend, make sure the format specifier in the dprintk()
at the top of nfsd_proc_write() is adjusted accordingly.


> Fixes: a51b5b737a0b ("NFSD: Update the NFSv2 WRITE argument decoder to use struct xdr_stream")
> Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
> ---
> From static analysis and I am not sure of the implications of this bug.

xdr_stream_subsegment() takes an unsigned int as its third
parameter. A large out-of-bounds value would cause it to return
false, so this bug should have no impact.

The use of "int" for the nfsd_writeargs::len field goes back
to before the git era, so I suppose just "Cc: stable" is adequate.


> fs/nfsd/nfsxdr.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/fs/nfsd/nfsxdr.c b/fs/nfsd/nfsxdr.c
> index aba8520b4b8b..a9f80e30320e 100644
> --- a/fs/nfsd/nfsxdr.c
> +++ b/fs/nfsd/nfsxdr.c
> @@ -336,7 +336,7 @@ nfssvc_decode_writeargs(struct svc_rqst *rqstp, struct xdr_stream *xdr)
> 	/* opaque data */
> 	if (xdr_stream_decode_u32(xdr, &args->len) < 0)
> 		return false;
> -	if (args->len > NFSSVC_MAXBLKSIZE_V2)
> +	if (args->len < 0 || args->len > NFSSVC_MAXBLKSIZE_V2)
> 		return false;
> 	if (!xdr_stream_subsegment(xdr, &args->payload, args->len))
> 		return false;
> -- 
> 2.20.1
> 

--
Chuck Lever

Re: [PATCH] NFSD: check for negative "len" values in nfssvc_decode_writeargs()

[Dan Carpenter]

On Mon, Mar 14, 2022 at 05:42:58PM +0300, Chuck Lever III wrote:
> Hi Dan-
> 
> > On Mar 14, 2022, at 10:06 AM, Dan Carpenter <dan.carpenter@oracle.com> wrote:
> > 
> > This code checks the upper bound of "len" but it needs to check for
> > negative values as well.
> 
> It doesn't check because nfsd3_writeargs::len is a __u32,
> and the NFSv2 code here was copied from that assuming that
> nfsd_writeargs::len had the same signage. This is because...
> 
> https://datatracker.ietf.org/doc/html/rfc1832#section-3.13 says
> that the count field in a variable-length array is supposed to
> be unsigned.
> 
> Thus IMO nfsd_writeargs::len should be changed to __u32
> instead of adding the extra negativity check.
> 
> If you resend, make sure the format specifier in the dprintk()
> at the top of nfsd_proc_write() is adjusted accordingly.

Thanks for this tip.  It's weird that GCC doesn't complain if you don't
make this change to the printk.  :/

regards,
dan carpenter