On Wed, Feb 21, 2024 at 02:46:21PM -0700, Nathan Chancellor wrote: > 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@xxxxxxxxxxxxxxx > Link: https://github.com/ClangBuiltLinux/linux/issues/1985 > Signed-off-by: Nathan Chancellor <nathan@xxxxxxxxxx> This seems reasonable to me. Thanks for chasing all this down! Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx> -Kees > --- > 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@xxxxxxxxxx> > -- Kees Cook
