linux-hardening - [PATCH 08/82] iov_iter: Avoid wrap-around instrumentation in copy_compat_iovec_from

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Date: Mon, 22 Jan 2024 16:26:43 -0800
From: Kees Cook <keescook@...omium.org>
To: linux-hardening@...r.kernel.org
Cc: Kees Cook <keescook@...omium.org>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Gustavo A. R. Silva" <gustavoars@...nel.org>,
	Bill Wendling <morbo@...gle.com>,
	Justin Stitt <justinstitt@...gle.com>,
	linux-kernel@...r.kernel.org
Subject: [PATCH 08/82] iov_iter: Avoid wrap-around instrumentation in copy_compat_iovec_from_user

The loop counter "i" in copy_compat_iovec_from_user() is an int, but
because the nr_segs argument is unsigned long, the signed overflow
sanitizer got worried "i" could wrap around. Instead of making "i" an
unsigned long (which may enlarge the type size), switch both nr_segs
and i to u32. There is no truncation with nr_segs since its is never
larger than UIO_MAXIOV anyway. This keeps sanitizer instrumentation[1]
out of a UACCESS path:

vmlinux.o: warning: objtool: copy_compat_iovec_from_user+0xa9: call to __ubsan_handle_add_overflow() with UACCESS enabled

Link: https://github.com/KSPP/linux/issues/26 [1]
Cc: Alexander Viro <viro@...iv.linux.org.uk>
Cc: Andrew Morton <akpm@...ux-foundation.org>
Signed-off-by: Kees Cook <keescook@...omium.org>
---
 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 e0aa6b440ca5..d797a43dca91 100644
--- a/lib/iov_iter.c
+++ b/lib/iov_iter.c
@@ -1166,11 +1166,12 @@ const void *dup_iter(struct iov_iter *new, struct iov_iter *old, gfp_t flags)
 EXPORT_SYMBOL(dup_iter);
 
 static __noclone int copy_compat_iovec_from_user(struct iovec *iov,
-		const struct iovec __user *uvec, unsigned long nr_segs)
+		const struct iovec __user *uvec, u32 nr_segs)
 {
 	const struct compat_iovec __user *uiov =
 		(const struct compat_iovec __user *)uvec;
-	int ret = -EFAULT, i;
+	int ret = -EFAULT;
+	u32 i;
 
 	if (!user_access_begin(uiov, nr_segs * sizeof(*uiov)))
 		return -EFAULT;
-- 
2.34.1