nfsd: don't replace page in rq_pages if it's a continuation of last page

Vulnerability Description

In the Linux kernel, the following vulnerability has been resolved: nfsd: don't replace page in rq_pages if it's a continuation of last page The splice read calls nfsd_splice_actor to put the pages containing file data into the svc_rqst->rq_pages array. It's possible however to get a splice result that only has a partial page at the end, if (e.g.) the filesystem hands back a short read that doesn't cover the whole page. nfsd_splice_actor will plop the partial page into its rq_pages array and return. Then later, when nfsd_splice_actor is called again, the remainder of the page may end up being filled out. At this point, nfsd_splice_actor will put the page into the array _again_ corrupting the reply. If this is done enough times, rq_next_page will overrun the array and corrupt the trailing fields -- the rq_respages and rq_next_page pointers themselves. If we've already added the page to the array in the last pass, don't add it to the array a second time when dealing with a splice continuation. This was originally handled properly in nfsd_splice_actor, but commit 91e23b1c3982 ("NFSD: Clean up nfsd_splice_actor()") removed the check for it.

References

• https://git.kernel.org/stable/c/8235cd619db6e67f1d7d26c55f1f3e4e575c947d
• https://git.kernel.org/stable/c/12eca509234acb6b666802edf77408bb70d7bfca
• https://git.kernel.org/stable/c/51ddb84baff6f09ad62b5999ece3ec172e4e3568
• https://git.kernel.org/stable/c/0101067f376eb7b9afd00279270f25d5111a091d
• https://git.kernel.org/stable/c/27c934dd8832dd40fd34776f916dc201e18b319b