Vulnerability: mm/filemap.c in the Linux kernel before 2.6.25 allows local users to cause a denial of service (infinite loop) via a writev system call that triggers an iovec of zero length, followed by a page fault for an iovec of nonzero length.
Vulnerability ID: CVE-2008-7316
CWE: CWE-20
Severity score: 2.9
Project: https://github.com/torvalds/linux
Published: 2016-05-02
File: mm/filemap.c
Commit message: fix writev regression: pan hanging unkillable and un-straceable

Frederik Himpe reported an unkillable and un-straceable pan process.

Zero length iovecs can go into an infinite loop in writev, because the
iovec iterator does not always advance over them.

The sequence required to trigger this is not trivial. I think it
requires that a zero-length iovec be followed by a non-zero-length iovec
which causes a pagefault in the atomic usercopy. This causes the writev
code to drop back into single-segment copy mode, which then tries to
copy the 0 bytes of the zero-length iovec; a zero length copy looks like
a failure though, so it loops.

Put a test into iov_iter_advance to catch zero-length iovecs. We could
just put the test in the fallback path, but I feel it is more robust to
skip over zero-length iovecs throughout the code (iovec iterator may be
used in filesystems too, so it should be robust).

Signed-off-by: Nick Piggin <npiggin@suse.de>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Commit author: Nick Piggin

Vulnerable version (before fix):
 static void __iov_iter_advance_iov(struct iov_iter *i, size_t bytes) 		const struct iovec *iov = i->iov; 		size_t base = i->iov_offset;  		while (bytes) { 			int copy = min(bytes, iov->iov_len - base);  			bytes -= copy; static ssize_t generic_perform_write(struct file *file,  		cond_resched();  		if (unlikely(copied == 0)) { 			/* 			 * If we were unable to copy any data at all, we must static ssize_t generic_perform_write(struct file *file, 						iov_iter_single_seg_count(i)); 			goto again; 		} 		iov_iter_advance(i, copied); 		pos += copied; 		written += copied; 

Fixed version (after fix):
 static void __iov_iter_advance_iov(struct iov_iter *i, size_t bytes) 		const struct iovec *iov = i->iov; 		size_t base = i->iov_offset;  		/* 		 * The !iov->iov_len check ensures we skip over unlikely 		 * zero-length segments. 		 */ 		while (bytes || !iov->iov_len) { 			int copy = min(bytes, iov->iov_len - base);  			bytes -= copy; static ssize_t generic_perform_write(struct file *file,  		cond_resched();  		iov_iter_advance(i, copied); 		if (unlikely(copied == 0)) { 			/* 			 * If we were unable to copy any data at all, we must static ssize_t generic_perform_write(struct file *file, 						iov_iter_single_seg_count(i)); 			goto again; 		} 		pos += copied; 		written += copied; 

Patch:
patch: @@ -1750,7 +1750,11 @@ static void __iov_iter_advance_iov(struct iov_iter *i, size_t bytes)
 		const struct iovec *iov = i->iov;
 		size_t base = i->iov_offset;
 
-		while (bytes) {
+		/*
+		 * The !iov->iov_len check ensures we skip over unlikely
+		 * zero-length segments.
+		 */
+		while (bytes || !iov->iov_len) {
 			int copy = min(bytes, iov->iov_len - base);
 
 			bytes -= copy;
@@ -2268,6 +2272,7 @@ static ssize_t generic_perform_write(struct file *file,
 
 		cond_resched();
 
+		iov_iter_advance(i, copied);
 		if (unlikely(copied == 0)) {
 			/*
 			 * If we were unable to copy any data at all, we must
@@ -2281,7 +2286,6 @@ static ssize_t generic_perform_write(struct file *file,
 						iov_iter_single_seg_count(i));
 			goto again;
 		}
-		iov_iter_advance(i, copied);
 		pos += copied;
 		written += copied;
path: mm/filemap.c