Re: [PATCH v5 33/37] s390/uaccess: Add KMSAN support to put_user() and get_user()

Alexander Potapenko
2024-06-20T08:36:51Z

On Wed, Jun 19, 2024 at 5:45 PM Ilya Leoshkevich <iii@linux.ibm.com> wrote:
>
> put_user() uses inline assembly with precise constraints, so Clang is
> in principle capable of instrumenting it automatically. Unfortunately,
> one of the constraints contains a dereferenced user pointer, and Clang
> does not currently distinguish user and kernel pointers. Therefore
> KMSAN attempts to access shadow for user pointers, which is not a right
> thing to do.

By the way, how does this problem manifest?
I was expecting KMSAN to generate dummy shadow accesses in this case,
and reading/writing 1-8 bytes from dummy shadow shouldn't be a
problem.

(On the other hand, not inlining the get_user/put_user functions is
probably still faster than retrieving the dummy shadow, so I'm fine
either way)

>
> An obvious fix to add __no_sanitize_memory to __put_user_fn() does not
> work, since it's __always_inline. And __always_inline cannot be removed
> due to the __put_user_bad() trick.
>
> A different obvious fix of using the "a" instead of the "+Q" constraint
> degrades the code quality, which is very important here, since it's a
> hot path.
>
> Instead, repurpose the __put_user_asm() macro to define
> __put_user_{char,short,int,long}_noinstr() functions and mark them with
> __no_sanitize_memory. For the non-KMSAN builds make them
> __always_inline in order to keep the generated code quality. Also
> define __put_user_{char,short,int,long}() functions, which call the
> aforementioned ones and which *are* instrumented, because they call
> KMSAN hooks, which may be implemented as macros.
>
> The same applies to get_user() as well.
>
> Acked-by: Heiko Carstens <hca@linux.ibm.com>
> Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
> ---
> arch/s390/include/asm/uaccess.h | 111 +++++++++++++++++++++++---------
> 1 file changed, 79 insertions(+), 32 deletions(-)
>
> diff --git a/arch/s390/include/asm/uaccess.h b/arch/s390/include/asm/uaccess.h
> index 81ae8a98e7ec..70f0edc00c2a 100644
> --- a/arch/s390/include/asm/uaccess.h
> +++ b/arch/s390/include/asm/uaccess.h
> @@ -78,13 +78,24 @@ union oac {
>
> int __noreturn __put_user_bad(void);
>
> -#define __put_user_asm(to, from, size) \
> -({ \
> +#ifdef CONFIG_KMSAN
> +#define get_put_user_noinstr_attributes \
> + noinline __maybe_unused __no_sanitize_memory
> +#else
> +#define get_put_user_noinstr_attributes __always_inline
> +#endif
> +
> +#define DEFINE_PUT_USER(type) \
> +static get_put_user_noinstr_attributes int \
> +__put_user_##type##_noinstr(unsigned type __user *to, \
> + unsigned type *from, \
> + unsigned long size) \
> +{ \
> union oac __oac_spec = { \
> .oac1.as = PSW_BITS_AS_SECONDARY, \
> .oac1.a = 1, \
> }; \
> - int __rc; \
> + int rc; \
> \
> asm volatile( \
> " lr 0,%[spec]\n" \
> @@ -93,12 +104,28 @@ int __noreturn __put_user_bad(void);
> "2:\n" \
> EX_TABLE_UA_STORE(0b, 2b, %[rc]) \
> EX_TABLE_UA_STORE(1b, 2b, %[rc]) \
> - : [rc] "=&d" (__rc), [_to] "+Q" (*(to)) \
> + : [rc] "=&d" (rc), [_to] "+Q" (*(to)) \
> : [_size] "d" (size), [_from] "Q" (*(from)), \
> [spec] "d" (__oac_spec.val) \
> : "cc", "0"); \
> - __rc; \
> -})
> + return rc; \
> +} \
> + \
> +static __always_inline int \
> +__put_user_##type(unsigned type __user *to, unsigned type *from, \
> + unsigned long size) \
> +{ \
> + int rc; \
> + \
> + rc = __put_user_##type##_noinstr(to, from, size); \
> + instrument_put_user(*from, to, size); \
> + return rc; \
> +}
> +
> +DEFINE_PUT_USER(char);
> +DEFINE_PUT_USER(short);
> +DEFINE_PUT_USER(int);
> +DEFINE_PUT_USER(long);
>
> static __always_inline int __put_user_fn(void *x, void __user *ptr, unsigned long size)
> {
> @@ -106,24 +133,24 @@ static __always_inline int __put_user_fn(void *x, void __user *ptr, unsigned lon
>
> switch (size) {
> case 1:
> - rc = __put_user_asm((unsigned char __user *)ptr,
> - (unsigned char *)x,
> - size);
> + rc = __put_user_char((unsigned char __user *)ptr,
> + (unsigned char *)x,
> + size);
> break;
> case 2:
> - rc = __put_user_asm((unsigned short __user *)ptr,
> - (unsigned short *)x,
> - size);
> + rc = __put_user_short((unsigned short __user *)ptr,
> + (unsigned short *)x,
> + size);
> break;
> case 4:
> - rc = __put_user_asm((unsigned int __user *)ptr,
> + rc = __put_user_int((unsigned int __user *)ptr,
> (unsigned int *)x,
> size);
> break;
> case 8:
> - rc = __put_user_asm((unsigned long __user *)ptr,
> - (unsigned long *)x,
> - size);
> + rc = __put_user_long((unsigned long __user *)ptr,
> + (unsigned long *)x,
> + size);
> break;
> default:
> __put_user_bad();
> @@ -134,13 +161,17 @@ static __always_inline int __put_user_fn(void *x, void __user *ptr, unsigned lon
>
> int __noreturn __get_user_bad(void);
>
> -#define __get_user_asm(to, from, size) \
> -({ \
> +#define DEFINE_GET_USER(type) \
> +static get_put_user_noinstr_attributes int \
> +__get_user_##type##_noinstr(unsigned type *to, \
> + unsigned type __user *from, \
> + unsigned long size) \
> +{ \
> union oac __oac_spec = { \
> .oac2.as = PSW_BITS_AS_SECONDARY, \
> .oac2.a = 1, \
> }; \
> - int __rc; \
> + int rc; \
> \
> asm volatile( \
> " lr 0,%[spec]\n" \
> @@ -149,13 +180,29 @@ int __noreturn __get_user_bad(void);
> "2:\n" \
> EX_TABLE_UA_LOAD_MEM(0b, 2b, %[rc], %[_to], %[_ksize]) \
> EX_TABLE_UA_LOAD_MEM(1b, 2b, %[rc], %[_to], %[_ksize]) \
> - : [rc] "=&d" (__rc), "=Q" (*(to)) \
> + : [rc] "=&d" (rc), "=Q" (*(to)) \
> : [_size] "d" (size), [_from] "Q" (*(from)), \
> [spec] "d" (__oac_spec.val), [_to] "a" (to), \
> [_ksize] "K" (size) \
> : "cc", "0"); \
> - __rc; \
> -})
> + return rc; \
> +} \
> + \
> +static __always_inline int \
> +__get_user_##type(unsigned type *to, unsigned type __user *from, \
> + unsigned long size) \
> +{ \
> + int rc; \
> + \
> + rc = __get_user_##type##_noinstr(to, from, size); \
> + instrument_get_user(*to); \
> + return rc; \
> +}
> +
> +DEFINE_GET_USER(char);
> +DEFINE_GET_USER(short);
> +DEFINE_GET_USER(int);
> +DEFINE_GET_USER(long);
>
> static __always_inline int __get_user_fn(void *x, const void __user *ptr, unsigned long size)
> {
> @@ -163,24 +210,24 @@ static __always_inline int __get_user_fn(void *x, const void __user *ptr, unsign
>
> switch (size) {
> case 1:
> - rc = __get_user_asm((unsigned char *)x,
> - (unsigned char __user *)ptr,
> - size);
> + rc = __get_user_char((unsigned char *)x,
> + (unsigned char __user *)ptr,
> + size);
> break;
> case 2:
> - rc = __get_user_asm((unsigned short *)x,
> - (unsigned short __user *)ptr,
> - size);
> + rc = __get_user_short((unsigned short *)x,
> + (unsigned short __user *)ptr,
> + size);
> break;
> case 4:
> - rc = __get_user_asm((unsigned int *)x,
> + rc = __get_user_int((unsigned int *)x,
> (unsigned int __user *)ptr,
> size);
> break;
> case 8:
> - rc = __get_user_asm((unsigned long *)x,
> - (unsigned long __user *)ptr,
> - size);
> + rc = __get_user_long((unsigned long *)x,
> + (unsigned long __user *)ptr,
> + size);
> break;
> default:
> __get_user_bad();
> --
> 2.45.1
>
> --
> You received this message because you are subscribed to the Google Groups "kasan-dev" group.
> To unsubscribe from this group and stop receiving emails from it, send an email to kasan-dev+unsubscribe@googlegroups.com.
> To view this discussion on the web visit https://groups.google.com/d/msgid/kasan-dev/20240619154530.163232-34-iii%40linux.ibm.com.

--
Alexander Potapenko
Software Engineer

Google Germany GmbH
Erika-Mann-Straße, 33
80636 München

Geschäftsführer: Paul Manicle, Liana Sebastian
Registergericht und -nummer: Hamburg, HRB 86891
Sitz der Gesellschaft: Hamburg