Re: [PATCH v3 net] ipvs: fix UB due to uninitialized stack access in ip_vs_protocol_init()

[Julian Anastasov <ja@ssi.bg>]

	Hello,

On Sat, 23 Nov 2024, Jinghao Jia wrote:

> Under certain kernel configurations when building with Clang/LLVM, the
> compiler does not generate a return or jump as the terminator
> instruction for ip_vs_protocol_init(), triggering the following objtool
> warning during build time:
> 
>   vmlinux.o: warning: objtool: ip_vs_protocol_init() falls through to next function __initstub__kmod_ip_vs_rr__935_123_ip_vs_rr_init6()
> 
> At runtime, this either causes an oops when trying to load the ipvs
> module or a boot-time panic if ipvs is built-in. This same issue has
> been reported by the Intel kernel test robot previously.
> 
> Digging deeper into both LLVM and the kernel code reveals this to be a
> undefined behavior problem. ip_vs_protocol_init() uses a on-stack buffer
> of 64 chars to store the registered protocol names and leaves it
> uninitialized after definition. The function calls strnlen() when
> concatenating protocol names into the buffer. With CONFIG_FORTIFY_SOURCE
> strnlen() performs an extra step to check whether the last byte of the
> input char buffer is a null character (commit 3009f891bb9f ("fortify:
> Allow strlen() and strnlen() to pass compile-time known lengths")).
> This, together with possibly other configurations, cause the following
> IR to be generated:
> 
>   define hidden i32 @ip_vs_protocol_init() local_unnamed_addr #5 section ".init.text" align 16 !kcfi_type !29 {
>     %1 = alloca [64 x i8], align 16
>     ...
> 
>   14:                                               ; preds = %11
>     %15 = getelementptr inbounds i8, ptr %1, i64 63
>     %16 = load i8, ptr %15, align 1
>     %17 = tail call i1 @llvm.is.constant.i8(i8 %16)
>     %18 = icmp eq i8 %16, 0
>     %19 = select i1 %17, i1 %18, i1 false
>     br i1 %19, label %20, label %23
> 
>   20:                                               ; preds = %14
>     %21 = call i64 @strlen(ptr noundef nonnull dereferenceable(1) %1) #23
>     ...
> 
>   23:                                               ; preds = %14, %11, %20
>     %24 = call i64 @strnlen(ptr noundef nonnull dereferenceable(1) %1, i64 noundef 64) #24
>     ...
>   }
> 
> The above code calculates the address of the last char in the buffer
> (value %15) and then loads from it (value %16). Because the buffer is
> never initialized, the LLVM GVN pass marks value %16 as undefined:
> 
>   %13 = getelementptr inbounds i8, ptr %1, i64 63
>   br i1 undef, label %14, label %17
> 
> This gives later passes (SCCP, in particular) more DCE opportunities by
> propagating the undef value further, and eventually removes everything
> after the load on the uninitialized stack location:
> 
>   define hidden i32 @ip_vs_protocol_init() local_unnamed_addr #0 section ".init.text" align 16 !kcfi_type !11 {
>     %1 = alloca [64 x i8], align 16
>     ...
> 
>   12:                                               ; preds = %11
>     %13 = getelementptr inbounds i8, ptr %1, i64 63
>     unreachable
>   }
> 
> In this way, the generated native code will just fall through to the
> next function, as LLVM does not generate any code for the unreachable IR
> instruction and leaves the function without a terminator.
> 
> Zero the on-stack buffer to avoid this possible UB.
> 
> Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
> Reported-by: kernel test robot <lkp@intel.com>
> Closes: https://lore.kernel.org/oe-kbuild-all/202402100205.PWXIz1ZK-lkp@intel.com/
> Co-developed-by: Ruowen Qin <ruqin@redhat.com>
> Signed-off-by: Ruowen Qin <ruqin@redhat.com>
> Signed-off-by: Jinghao Jia <jinghao7@illinois.edu>

	Looks good to me, thanks!

Acked-by: Julian Anastasov <ja@ssi.bg>

> ---
> Changelog:
> v2 -> v3:
> v2: https://lore.kernel.org/lkml/20241122045257.27452-1-jinghao7@illinois.edu/
> * Fix changelog format based on Julian's feedback
> 
> v1 -> v2:
> v1: https://lore.kernel.org/lkml/20241111065105.82431-1-jinghao7@illinois.edu/
> * Fix small error in commit message
> * Address Julian's feedback:
>   * Make this patch target the net tree rather than net-next
>   * Add a "Fixes" tag for the initial git commit
> 
>  net/netfilter/ipvs/ip_vs_proto.c | 4 +---
>  1 file changed, 1 insertion(+), 3 deletions(-)
> 
> diff --git a/net/netfilter/ipvs/ip_vs_proto.c b/net/netfilter/ipvs/ip_vs_proto.c
> index f100da4ba3bc..a9fd1d3fc2cb 100644
> --- a/net/netfilter/ipvs/ip_vs_proto.c
> +++ b/net/netfilter/ipvs/ip_vs_proto.c
> @@ -340,7 +340,7 @@ void __net_exit ip_vs_protocol_net_cleanup(struct netns_ipvs *ipvs)
>  
>  int __init ip_vs_protocol_init(void)
>  {
> -	char protocols[64];
> +	char protocols[64] = { 0 };
>  #define REGISTER_PROTOCOL(p)			\
>  	do {					\
>  		register_ip_vs_protocol(p);	\
> @@ -348,8 +348,6 @@ int __init ip_vs_protocol_init(void)
>  		strcat(protocols, (p)->name);	\
>  	} while (0)
>  
> -	protocols[0] = '\0';
> -	protocols[2] = '\0';
>  #ifdef CONFIG_IP_VS_PROTO_TCP
>  	REGISTER_PROTOCOL(&ip_vs_protocol_tcp);
>  #endif
> -- 
> 2.47.0

Regards

--
Julian Anastasov <ja@ssi.bg>