Hello all, I have discovered a signed/unsigned issue in a routine responsible for demarshalling XDR data for NFSv3 procedure calls. As far as I can tell, this bug has existed since NFSv3 support was integrated. It has been silently fixed in 2.4.21. The bug is in the decode_fh routine of fs/nfsd/nfs3xdr.c under the kernel source tree. Vulnerable code: static inline u32 * decode_fh(u32 *p, struct svc_fh *fhp) { int size; fh_init(fhp, NFS3_FHSIZE); size = ntohl(*p++); if (size > NFS3_FHSIZE) return NULL; memcpy(&fhp->fh_handle.fh_base, p, size); fhp->fh_handle.fh_size = size; return p + XDR_QUADLEN(size); } Where p is a packet of attacker controlled XDR data. If size is made to be negative, the sanity check is passed and the malicious value is passed to memcpy. Due to the behavior of the kernel's memcpy, this will cause a very large copy in kernel space, resulting in an instant kernel panic. The attached code is a POC of this vulnerability. It requires that the vulnerable host has an exported directory available to the attacker. This is probably not the only way to manifest this bug, however. If you have any questions, please feel free to contact me. Cheers, Jared Stanbrough <jaredsat_private>
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