linux-kernel - [PATCH] iov_iter: Fix iov_iter_get_pages{,

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Date:   Wed, 21 Jul 2021 23:01:45 +0200
From:   Andreas Gruenbacher <agruenba@...hat.com>
To:     Alexander Viro <viro@...iv.linux.org.uk>,
        Steve French <sfrench@...ba.org>,
        Trond Myklebust <trond.myklebust@...merspace.com>,
        Anna Schumaker <anna.schumaker@...app.com>
Cc:     Andreas Gruenbacher <agruenba@...hat.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-cifs@...r.kernel.org
Subject: [PATCH] iov_iter: Fix iov_iter_get_pages{,_alloc} page fault return value

Both iov_iter_get_pages and iov_iter_get_pages_alloc return the number of bytes
of the iovec they could get the pages for.  When they cannot get any pages,
they're supposed to return 0, but when the start of the iovec isn't page
aligned, the calculation goes wrong and they return a negative value.
Fix that in both functions.

In addition, change iov_iter_get_pages_alloc to return ZERO_SIZE_PTR in that
case to prevent resource leaks.

It seems that the cifs and nfs filesystems don't handle the zero case very
well.  Steve, Trond, Anna, could you please have a look?

Signed-off-by: Andreas Gruenbacher <agruenba@...hat.com>
---
 lib/iov_iter.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/lib/iov_iter.c b/lib/iov_iter.c
index e23123ae3a13..20dc3d800573 100644
--- a/lib/iov_iter.c
+++ b/lib/iov_iter.c
@@ -1484,7 +1484,7 @@ ssize_t iov_iter_get_pages(struct iov_iter *i,
 		res = get_user_pages_fast(addr, n,
 				iov_iter_rw(i) != WRITE ?  FOLL_WRITE : 0,
 				pages);
-		if (unlikely(res < 0))
+		if (unlikely(res <= 0))
 			return res;
 		return (res == n ? len : res * PAGE_SIZE) - *start;
 	}
@@ -1608,8 +1608,9 @@ ssize_t iov_iter_get_pages_alloc(struct iov_iter *i,
 			return -ENOMEM;
 		res = get_user_pages_fast(addr, n,
 				iov_iter_rw(i) != WRITE ?  FOLL_WRITE : 0, p);
-		if (unlikely(res < 0)) {
+		if (unlikely(res <= 0)) {
 			kvfree(p);
+			*pages = ZERO_SIZE_PTR;
 			return res;
 		}
 		*pages = p;
-- 
2.26.3