netdev - [PATCH net] xfrm: Avoid clang fortify warning in copy_to_user

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Message-Id: <20240221-xfrm-avoid-clang-fortify-warning-copy_to_user_tmpl-v1-1-254a788ab8ba@kernel.org>
Date: Wed, 21 Feb 2024 14:46:21 -0700
From: Nathan Chancellor <nathan@...nel.org>
To: steffen.klassert@...unet.com, herbert@...dor.apana.org.au, 
 davem@...emloft.net
Cc: edumazet@...gle.com, kuba@...nel.org, pabeni@...hat.com, 
 morbo@...gle.com, justinstitt@...gle.com, keescook@...omium.org, 
 netdev@...r.kernel.org, llvm@...ts.linux.dev, patches@...ts.linux.dev, 
 stable@...r.kernel.org, Nathan Chancellor <nathan@...nel.org>
Subject: [PATCH net] xfrm: Avoid clang fortify warning in
 copy_to_user_tmpl()

After a couple recent changes in LLVM, there is a warning (or error with
CONFIG_WERROR=y or W=e) from the compile time fortify source routines,
specifically the memset() in copy_to_user_tmpl().

  In file included from net/xfrm/xfrm_user.c:14:
  ...
  include/linux/fortify-string.h:438:4: error: call to '__write_overflow_field' declared with 'warning' attribute: detected write beyond size of field (1st parameter); maybe use struct_group()? [-Werror,-Wattribute-warning]
    438 |                         __write_overflow_field(p_size_field, size);
        |                         ^
  1 error generated.

While ->xfrm_nr has been validated against XFRM_MAX_DEPTH when its value
is first assigned in copy_templates() by calling validate_tmpl() first
(so there should not be any issue in practice), LLVM/clang cannot really
deduce that across the boundaries of these functions. Without that
knowledge, it cannot assume that the loop stops before i is greater than
XFRM_MAX_DEPTH, which would indeed result a stack buffer overflow in the
memset().

To make the bounds of ->xfrm_nr clear to the compiler and add additional
defense in case copy_to_user_tmpl() is ever used in a path where
->xfrm_nr has not been properly validated against XFRM_MAX_DEPTH first,
add an explicit bound check and early return, which clears up the
warning.

Cc: stable@...r.kernel.org
Link: https://github.com/ClangBuiltLinux/linux/issues/1985
Signed-off-by: Nathan Chancellor <nathan@...nel.org>
---
 net/xfrm/xfrm_user.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/net/xfrm/xfrm_user.c b/net/xfrm/xfrm_user.c
index f037be190bae..912c1189ba41 100644
--- a/net/xfrm/xfrm_user.c
+++ b/net/xfrm/xfrm_user.c
@@ -2017,6 +2017,9 @@ static int copy_to_user_tmpl(struct xfrm_policy *xp, struct sk_buff *skb)
 	if (xp->xfrm_nr == 0)
 		return 0;
 
+	if (xp->xfrm_nr > XFRM_MAX_DEPTH)
+		return -ENOBUFS;
+
 	for (i = 0; i < xp->xfrm_nr; i++) {
 		struct xfrm_user_tmpl *up = &vec[i];
 		struct xfrm_tmpl *kp = &xp->xfrm_vec[i];

---
base-commit: 14dec56fdd4c70a0ebe40077368e367421ea6fef
change-id: 20240221-xfrm-avoid-clang-fortify-warning-copy_to_user_tmpl-40cb10b003e3

Best regards,
-- 
Nathan Chancellor <nathan@...nel.org>