[PATCH] bug in read_buf

[Neil Brown <neilb@suse.de>]

Surely this can never have worked... which implies that the code has
never been used?

When read_buf is called to move over to the next page in the pagelist
of an NFSv4 request, it sets argp->end to essentially a random
number, certainly not an address within the page which argp->p now
points to.  So subsequent calls to READ_BUF will think there is much
more than a page of spare space (the cast to u32 ensures an unsigned
comparison) so we can expect to fall off the end of the second
page.

I guess we never ever receive requests with any operation starting
beyond the first page!

[[
I found this while looking at why fsstress over NFS over RDMA caused
a bad memory dereference in READ32, suggesting that 'p' had a bad
value.  However it was ffff8801299188f0, which is not an "I've fallen
off the end of the page" sort of value.  So I think it must be a
different bug :-(  It is as if the page is being unmapped underneath
us...
]]
NeilBrown




diff --git a/fs/nfsd/nfs4xdr.c b/fs/nfsd/nfs4xdr.c
index e170317..34ccf81 100644
--- a/fs/nfsd/nfs4xdr.c
+++ b/fs/nfsd/nfs4xdr.c
@@ -161,10 +161,10 @@ static __be32 *read_buf(struct nfsd4_compoundargs *argp, u32 nbytes)
 	argp->p = page_address(argp->pagelist[0]);
 	argp->pagelist++;
 	if (argp->pagelen < PAGE_SIZE) {
-		argp->end = p + (argp->pagelen>>2);
+		argp->end = argp->p + (argp->pagelen>>2);
 		argp->pagelen = 0;
 	} else {
-		argp->end = p + (PAGE_SIZE>>2);
+		argp->end = argp->p + (PAGE_SIZE>>2);
 		argp->pagelen -= PAGE_SIZE;
 	}
 	memcpy(((char*)p)+avail, argp->p, (nbytes - avail));
@@ -1426,10 +1426,10 @@ nfsd4_decode_compound(struct nfsd4_compoundargs *argp)
 			argp->p = page_address(argp->pagelist[0]);
 			argp->pagelist++;
 			if (argp->pagelen < PAGE_SIZE) {
-				argp->end = p + (argp->pagelen>>2);
+				argp->end = argp->p + (argp->pagelen>>2);
 				argp->pagelen = 0;
 			} else {
-				argp->end = p + (PAGE_SIZE>>2);
+				argp->end = argp->p + (PAGE_SIZE>>2);
 				argp->pagelen -= PAGE_SIZE;
 			}
 		}