[PATCH v4 3/4] x86/uaccess: Use pointer masking to limit uaccess speculation

Josh Poimboeuf
2021-05-05T03:55:05Z

The x86 uaccess code uses barrier_nospec() in various places to prevent
speculative dereferencing of user-controlled pointers (which might be
combined with further gadgets or CPU bugs to leak data).

There are some issues with the current implementation:

- The barrier_nospec() in copy_from_user() was inadvertently removed
with: 4b842e4e25b1 ("x86: get rid of small constant size cases in
raw_copy_{to,from}_user()")

- copy_to_user() and friends should also have a speculation barrier,
because a speculative write to a user-controlled address can still
populate the cache line with the original data.

- The LFENCE in barrier_nospec() is overkill, when more lightweight user
pointer masking can be used instead.

Remove existing barrier_nospec() usage, and instead do user pointer
masking, throughout the x86 uaccess code. This is similar to what arm64
is already doing with uaccess_mask_ptr().

Fixes: 4b842e4e25b1 ("x86: get rid of small constant size cases in raw_copy_{to,from}_user()")
Suggested-by: Will Deacon <will@kernel.org>
Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
---
Documentation/admin-guide/hw-vuln/spectre.rst | 6 +--
arch/x86/include/asm/futex.h | 5 ++
arch/x86/include/asm/uaccess.h | 46 +++++++++++++------
arch/x86/include/asm/uaccess_64.h | 12 ++---
arch/x86/kernel/cpu/sgx/virt.c | 6 ++-
arch/x86/lib/copy_mc.c | 2 +
arch/x86/lib/csum-wrappers_64.c | 5 +-
arch/x86/lib/getuser.S | 16 ++-----
arch/x86/lib/putuser.S | 8 ++++
arch/x86/lib/usercopy_32.c | 6 +--
arch/x86/lib/usercopy_64.c | 7 +--
11 files changed, 76 insertions(+), 43 deletions(-)

diff --git a/Documentation/admin-guide/hw-vuln/spectre.rst b/Documentation/admin-guide/hw-vuln/spectre.rst
index e05e581af5cf..2348d27d61da 100644
--- a/Documentation/admin-guide/hw-vuln/spectre.rst
+++ b/Documentation/admin-guide/hw-vuln/spectre.rst
@@ -426,9 +426,9 @@ Spectre variant 1
<spec_ref2>` to avoid any usable disclosure gadgets. However, it may
not cover all attack vectors for Spectre variant 1.

- Copy-from-user code has an LFENCE barrier to prevent the access_ok()
- check from being mis-speculated. The barrier is done by the
- barrier_nospec() macro.
+ Usercopy code uses user pointer masking to prevent the access_ok()
+ check from being mis-speculated in the success path with a kernel
+ address. The masking is done by the mask_user_ptr() macro.

For the swapgs variant of Spectre variant 1, LFENCE barriers are
added to interrupt, exception and NMI entry where needed. These
diff --git a/arch/x86/include/asm/futex.h b/arch/x86/include/asm/futex.h
index f9c00110a69a..6224b2f15a0f 100644
--- a/arch/x86/include/asm/futex.h
+++ b/arch/x86/include/asm/futex.h
@@ -59,6 +59,8 @@ static __always_inline int arch_futex_atomic_op_inuser(int op, int oparg, int *o
if (!user_access_begin(uaddr, sizeof(u32)))
return -EFAULT;

+ uaddr = mask_user_ptr(uaddr);
+
switch (op) {
case FUTEX_OP_SET:
unsafe_atomic_op1("xchgl %0, %2", oval, uaddr, oparg, Efault);
@@ -94,6 +96,9 @@ static inline int futex_atomic_cmpxchg_inatomic(u32 *uval, u32 __user *uaddr,

if (!user_access_begin(uaddr, sizeof(u32)))
return -EFAULT;
+
+ uaddr = mask_user_ptr(uaddr);
+
asm volatile("\n"
"1:\t" LOCK_PREFIX "cmpxchgl %4, %2\n"
"2:\n"
diff --git a/arch/x86/include/asm/uaccess.h b/arch/x86/include/asm/uaccess.h
index fb75657b5e56..ebe9ab46b183 100644
--- a/arch/x86/include/asm/uaccess.h
+++ b/arch/x86/include/asm/uaccess.h
@@ -66,12 +66,35 @@ static inline bool pagefault_disabled(void);
* Return: true (nonzero) if the memory block may be valid, false (zero)
* if it is definitely invalid.
*/
-#define access_ok(addr, size) \
+#define access_ok(addr, size) \
({ \
WARN_ON_IN_IRQ(); \
likely(!__range_not_ok(addr, size, TASK_SIZE_MAX)); \
})

+/*
+ * Sanitize a user pointer such that it becomes NULL if it's not a valid user
+ * pointer. This prevents speculatively dereferencing a user-controlled
+ * pointer to kernel space if access_ok() speculatively returns true. This
+ * should be done *after* access_ok(), to avoid affecting error handling
+ * behavior.
+ */
+#define mask_user_ptr(ptr) \
+({ \
+ unsigned long _ptr = (__force unsigned long)ptr; \
+ unsigned long mask; \
+ \
+ asm volatile("cmp %[max], %[_ptr]\n\t" \
+ "sbb %[mask], %[mask]\n\t" \
+ : [mask] "=r" (mask) \
+ : [_ptr] "r" (_ptr), \
+ [max] "r" (TASK_SIZE_MAX) \
+ : "cc"); \
+ \
+ mask &= _ptr; \
+ ((typeof(ptr)) mask); \
+})
+
extern int __get_user_1(void);
extern int __get_user_2(void);
extern int __get_user_4(void);
@@ -84,11 +107,6 @@ extern int __get_user_bad(void);

#define __uaccess_begin() stac()
#define __uaccess_end() clac()
-#define __uaccess_begin_nospec() \
-({ \
- stac(); \
- barrier_nospec(); \
-})

/*
* This is the smallest unsigned integer type that can fit a value
@@ -175,7 +193,7 @@ extern int __get_user_bad(void);
* Return: zero on success, or -EFAULT on error.
* On error, the variable @x is set to zero.
*/
-#define __get_user(x,ptr) do_get_user_call(get_user_nocheck,x,ptr)
+#define __get_user(x,ptr) do_get_user_call(get_user_nocheck, x, mask_user_ptr(ptr))

#ifdef CONFIG_X86_32
@@ -271,7 +289,7 @@ extern void __put_user_nocheck_8(void);
*
* Return: zero on success, or -EFAULT on error.
*/
-#define __put_user(x, ptr) do_put_user_call(put_user_nocheck,x,ptr)
+#define __put_user(x, ptr) do_put_user_call(put_user_nocheck, x, mask_user_ptr(ptr))

#define __put_user_size(x, ptr, size, label) \
do { \
@@ -475,7 +493,7 @@ static __must_check __always_inline bool user_access_begin(const void __user *pt
{
if (unlikely(!access_ok(ptr,len)))
return 0;
- __uaccess_begin_nospec();
+ __uaccess_begin();
return 1;
}
#define user_access_begin(a,b) user_access_begin(a,b)
@@ -484,14 +502,15 @@ static __must_check __always_inline bool user_access_begin(const void __user *pt
#define user_access_save() smap_save()
#define user_access_restore(x) smap_restore(x)

-#define unsafe_put_user(x, ptr, label) \
- __put_user_size((__typeof__(*(ptr)))(x), (ptr), sizeof(*(ptr)), label)
+#define unsafe_put_user(x, ptr, label) \
+ __put_user_size((__typeof__(*(ptr)))(x), mask_user_ptr(ptr), \
+ sizeof(*(ptr)), label)

#ifdef CONFIG_CC_HAS_ASM_GOTO_OUTPUT
#define unsafe_get_user(x, ptr, err_label) \
do { \
__inttype(*(ptr)) __gu_val; \
- __get_user_size(__gu_val, (ptr), sizeof(*(ptr)), err_label); \
+ __get_user_size(__gu_val, mask_user_ptr(ptr), sizeof(*(ptr)), err_label);\
(x) = (__force __typeof__(*(ptr)))__gu_val; \
} while (0)
#else // !CONFIG_CC_HAS_ASM_GOTO_OUTPUT
@@ -499,7 +518,8 @@ do { \
do { \
int __gu_err; \
__inttype(*(ptr)) __gu_val; \
- __get_user_size(__gu_val, (ptr), sizeof(*(ptr)), __gu_err); \
+ __get_user_size(__gu_val, mask_user_ptr(ptr), sizeof(*(ptr)), \
+ __gu_err); \
(x) = (__force __typeof__(*(ptr)))__gu_val; \
if (unlikely(__gu_err)) goto err_label; \
} while (0)
diff --git a/arch/x86/include/asm/uaccess_64.h b/arch/x86/include/asm/uaccess_64.h
index e7265a552f4f..abd9cb204fde 100644
--- a/arch/x86/include/asm/uaccess_64.h
+++ b/arch/x86/include/asm/uaccess_64.h
@@ -49,20 +49,20 @@ copy_user_generic(void *to, const void *from, unsigned len)
static __always_inline __must_check unsigned long
raw_copy_from_user(void *dst, const void __user *src, unsigned long size)
{
- return copy_user_generic(dst, (__force void *)src, size);
+ return copy_user_generic(dst, (__force void *)mask_user_ptr(src), size);
}

static __always_inline __must_check unsigned long
raw_copy_to_user(void __user *dst, const void *src, unsigned long size)
{
- return copy_user_generic((__force void *)dst, src, size);
+ return copy_user_generic((__force void *)mask_user_ptr(dst), src, size);
}

static __always_inline __must_check
unsigned long raw_copy_in_user(void __user *dst, const void __user *src, unsigned long size)
{
- return copy_user_generic((__force void *)dst,
- (__force void *)src, size);
+ return copy_user_generic((__force void *)mask_user_ptr(dst),
+ (__force void *)mask_user_ptr(src), size);
}

extern long __copy_user_nocache(void *dst, const void __user *src,
@@ -77,13 +77,13 @@ __copy_from_user_inatomic_nocache(void *dst, const void __user *src,
unsigned size)
{
kasan_check_write(dst, size);
- return __copy_user_nocache(dst, src, size, 0);
+ return __copy_user_nocache(dst, mask_user_ptr(src), size, 0);
}

static inline int
__copy_from_user_flushcache(void *dst, const void __user *src, unsigned size)
{
kasan_check_write(dst, size);
- return __copy_user_flushcache(dst, src, size);
+ return __copy_user_flushcache(dst, mask_user_ptr(src), size);
}
#endif /* _ASM_X86_UACCESS_64_H */
diff --git a/arch/x86/kernel/cpu/sgx/virt.c b/arch/x86/kernel/cpu/sgx/virt.c
index 6ad165a5c0cc..1b6606afad36 100644
--- a/arch/x86/kernel/cpu/sgx/virt.c
+++ b/arch/x86/kernel/cpu/sgx/virt.c
@@ -292,7 +292,7 @@ int sgx_virt_ecreate(struct sgx_pageinfo *pageinfo, void __user *secs,
return -EINVAL;

__uaccess_begin();
- ret = __ecreate(pageinfo, (void *)secs);
+ ret = __ecreate(pageinfo, (void *)mask_user_ptr(secs));
__uaccess_end();

if (encls_faulted(ret)) {
@@ -323,7 +323,9 @@ static int __sgx_virt_einit(void __user *sigstruct, void __user *token,
return -EINVAL;

__uaccess_begin();
- ret = __einit((void *)sigstruct, (void *)token, (void *)secs);
+ ret = __einit((void *)mask_user_ptr(sigstruct),
+ (void *)mask_user_ptr(token),
+ (void *)mask_user_ptr(secs));
__uaccess_end();

return ret;
diff --git a/arch/x86/lib/copy_mc.c b/arch/x86/lib/copy_mc.c
index 6e8b7e600def..b895bafbe7fe 100644
--- a/arch/x86/lib/copy_mc.c
+++ b/arch/x86/lib/copy_mc.c
@@ -74,6 +74,8 @@ unsigned long __must_check copy_mc_to_user(void __user *dst, const void *src, un
{
unsigned long ret;

+ dst = mask_user_ptr(dst);
+
if (copy_mc_fragile_enabled) {
__uaccess_begin();
ret = copy_mc_fragile((__force void *)dst, src, len);
diff --git a/arch/x86/lib/csum-wrappers_64.c b/arch/x86/lib/csum-wrappers_64.c
index 189344924a2b..b022d34b9c4b 100644
--- a/arch/x86/lib/csum-wrappers_64.c
+++ b/arch/x86/lib/csum-wrappers_64.c
@@ -28,7 +28,8 @@ csum_and_copy_from_user(const void __user *src, void *dst, int len)
might_sleep();
if (!user_access_begin(src, len))
return 0;
- sum = csum_partial_copy_generic((__force const void *)src, dst, len);
+ sum = csum_partial_copy_generic((__force const void *)mask_user_ptr(src),
+ dst, len);
user_access_end();
return sum;
}
@@ -53,7 +54,7 @@ csum_and_copy_to_user(const void *src, void __user *dst, int len)
might_sleep();
if (!user_access_begin(dst, len))
return 0;
- sum = csum_partial_copy_generic(src, (void __force *)dst, len);
+ sum = csum_partial_copy_generic(src, (void __force *)mask_user_ptr(dst), len);
user_access_end();
return sum;
}
diff --git a/arch/x86/lib/getuser.S b/arch/x86/lib/getuser.S
index fa1bc2104b32..64715a7edb1f 100644
--- a/arch/x86/lib/getuser.S
+++ b/arch/x86/lib/getuser.S
@@ -35,8 +35,6 @@
#include <asm/smap.h>
#include <asm/export.h>

-#define ASM_BARRIER_NOSPEC ALTERNATIVE "", "lfence", X86_FEATURE_LFENCE_RDTSC
-
#ifdef CONFIG_X86_5LEVEL
#define LOAD_TASK_SIZE_MINUS_N(n) \
ALTERNATIVE __stringify(mov $((1 << 47) - 4096 - (n)),%rdx), \
@@ -51,7 +49,7 @@ SYM_FUNC_START(__get_user_1)
LOAD_TASK_SIZE_MINUS_N(0)
cmp %_ASM_DX,%_ASM_AX
jae bad_get_user
- sbb %_ASM_DX, %_ASM_DX /* array_index_mask_nospec() */
+ sbb %_ASM_DX, %_ASM_DX /* mask_user_ptr() */
and %_ASM_DX, %_ASM_AX
ASM_STAC
1: movzbl (%_ASM_AX),%edx
@@ -65,7 +63,7 @@ SYM_FUNC_START(__get_user_2)
LOAD_TASK_SIZE_MINUS_N(1)
cmp %_ASM_DX,%_ASM_AX
jae bad_get_user
- sbb %_ASM_DX, %_ASM_DX /* array_index_mask_nospec() */
+ sbb %_ASM_DX, %_ASM_DX /* mask_user_ptr() */
and %_ASM_DX, %_ASM_AX
ASM_STAC
2: movzwl (%_ASM_AX),%edx
@@ -79,7 +77,7 @@ SYM_FUNC_START(__get_user_4)
LOAD_TASK_SIZE_MINUS_N(3)
cmp %_ASM_DX,%_ASM_AX
jae bad_get_user
- sbb %_ASM_DX, %_ASM_DX /* array_index_mask_nospec() */
+ sbb %_ASM_DX, %_ASM_DX /* mask_user_ptr() */
and %_ASM_DX, %_ASM_AX
ASM_STAC
3: movl (%_ASM_AX),%edx
@@ -94,7 +92,7 @@ SYM_FUNC_START(__get_user_8)
LOAD_TASK_SIZE_MINUS_N(7)
cmp %_ASM_DX,%_ASM_AX
jae bad_get_user
- sbb %_ASM_DX, %_ASM_DX /* array_index_mask_nospec() */
+ sbb %_ASM_DX, %_ASM_DX /* mask_user_ptr() */
and %_ASM_DX, %_ASM_AX
ASM_STAC
4: movq (%_ASM_AX),%rdx
@@ -105,7 +103,7 @@ SYM_FUNC_START(__get_user_8)
LOAD_TASK_SIZE_MINUS_N(7)
cmp %_ASM_DX,%_ASM_AX
jae bad_get_user_8
- sbb %_ASM_DX, %_ASM_DX /* array_index_mask_nospec() */
+ sbb %_ASM_DX, %_ASM_DX /* mask_user_ptr() */
and %_ASM_DX, %_ASM_AX
ASM_STAC
4: movl (%_ASM_AX),%edx
@@ -120,7 +118,6 @@ EXPORT_SYMBOL(__get_user_8)
/* .. and the same for __get_user, just without the range checks */
SYM_FUNC_START(__get_user_nocheck_1)
ASM_STAC
- ASM_BARRIER_NOSPEC
6: movzbl (%_ASM_AX),%edx
xor %eax,%eax
ASM_CLAC
@@ -130,7 +127,6 @@ EXPORT_SYMBOL(__get_user_nocheck_1)

SYM_FUNC_START(__get_user_nocheck_2)
ASM_STAC
- ASM_BARRIER_NOSPEC
7: movzwl (%_ASM_AX),%edx
xor %eax,%eax
ASM_CLAC
@@ -140,7 +136,6 @@ EXPORT_SYMBOL(__get_user_nocheck_2)

SYM_FUNC_START(__get_user_nocheck_4)
ASM_STAC
- ASM_BARRIER_NOSPEC
8: movl (%_ASM_AX),%edx
xor %eax,%eax
ASM_CLAC
@@ -150,7 +145,6 @@ EXPORT_SYMBOL(__get_user_nocheck_4)

SYM_FUNC_START(__get_user_nocheck_8)
ASM_STAC
- ASM_BARRIER_NOSPEC
#ifdef CONFIG_X86_64
9: movq (%_ASM_AX),%rdx
#else
diff --git a/arch/x86/lib/putuser.S b/arch/x86/lib/putuser.S
index 0ea344c5ea43..afd819459455 100644
--- a/arch/x86/lib/putuser.S
+++ b/arch/x86/lib/putuser.S
@@ -47,6 +47,8 @@ SYM_FUNC_START(__put_user_1)
LOAD_TASK_SIZE_MINUS_N(0)
cmp %_ASM_BX,%_ASM_CX
jae .Lbad_put_user
+ sbb %_ASM_BX, %_ASM_BX /* mask_user_ptr() */
+ and %_ASM_BX, %_ASM_CX
SYM_INNER_LABEL(__put_user_nocheck_1, SYM_L_GLOBAL)
ASM_STAC
1: movb %al,(%_ASM_CX)
@@ -61,6 +63,8 @@ SYM_FUNC_START(__put_user_2)
LOAD_TASK_SIZE_MINUS_N(1)
cmp %_ASM_BX,%_ASM_CX
jae .Lbad_put_user
+ sbb %_ASM_BX, %_ASM_BX /* mask_user_ptr() */
+ and %_ASM_BX, %_ASM_CX
SYM_INNER_LABEL(__put_user_nocheck_2, SYM_L_GLOBAL)
ASM_STAC
2: movw %ax,(%_ASM_CX)
@@ -75,6 +79,8 @@ SYM_FUNC_START(__put_user_4)
LOAD_TASK_SIZE_MINUS_N(3)
cmp %_ASM_BX,%_ASM_CX
jae .Lbad_put_user
+ sbb %_ASM_BX, %_ASM_BX /* mask_user_ptr() */
+ and %_ASM_BX, %_ASM_CX
SYM_INNER_LABEL(__put_user_nocheck_4, SYM_L_GLOBAL)
ASM_STAC
3: movl %eax,(%_ASM_CX)
@@ -89,6 +95,8 @@ SYM_FUNC_START(__put_user_8)
LOAD_TASK_SIZE_MINUS_N(7)
cmp %_ASM_BX,%_ASM_CX
jae .Lbad_put_user
+ sbb %_ASM_BX, %_ASM_BX /* mask_user_ptr() */
+ and %_ASM_BX, %_ASM_CX
SYM_INNER_LABEL(__put_user_nocheck_8, SYM_L_GLOBAL)
ASM_STAC
4: mov %_ASM_AX,(%_ASM_CX)
diff --git a/arch/x86/lib/usercopy_32.c b/arch/x86/lib/usercopy_32.c
index 7d290777246d..e4dc3c2790db 100644
--- a/arch/x86/lib/usercopy_32.c
+++ b/arch/x86/lib/usercopy_32.c
@@ -68,7 +68,7 @@ clear_user(void __user *to, unsigned long n)
{
might_fault();
if (access_ok(to, n))
- __do_clear_user(to, n);
+ __do_clear_user(mask_user_ptr(to), n);
return n;
}
EXPORT_SYMBOL(clear_user);
@@ -331,7 +331,7 @@ do { \

unsigned long __copy_user_ll(void *to, const void *from, unsigned long n)
{
- __uaccess_begin_nospec();
+ __uaccess_begin();
if (movsl_is_ok(to, from, n))
__copy_user(to, from, n);
else
@@ -344,7 +344,7 @@ EXPORT_SYMBOL(__copy_user_ll);
unsigned long __copy_from_user_ll_nocache_nozero(void *to, const void __user *from,
unsigned long n)
{
- __uaccess_begin_nospec();
+ __uaccess_begin();
#ifdef CONFIG_X86_INTEL_USERCOPY
if (n > 64 && static_cpu_has(X86_FEATURE_XMM2))
n = __copy_user_intel_nocache(to, from, n);
diff --git a/arch/x86/lib/usercopy_64.c b/arch/x86/lib/usercopy_64.c
index 508c81e97ab1..be0e5efdd142 100644
--- a/arch/x86/lib/usercopy_64.c
+++ b/arch/x86/lib/usercopy_64.c
@@ -42,7 +42,8 @@ unsigned long __clear_user(void __user *addr, unsigned long size)
_ASM_EXTABLE_UA(0b, 3b)
_ASM_EXTABLE_UA(1b, 2b)
: [size8] "=&c"(size), [dst] "=&D" (__d0)
- : [size1] "r"(size & 7), "[size8]" (size / 8), "[dst]"(addr));
+ : [size1] "r"(size & 7), "[size8]" (size / 8),
+ "[dst]" (mask_user_ptr(addr)));
clac();
return size;
}
@@ -51,7 +52,7 @@ EXPORT_SYMBOL(__clear_user);
unsigned long clear_user(void __user *to, unsigned long n)
{
if (access_ok(to, n))
- return __clear_user(to, n);
+ return __clear_user(mask_user_ptr(to), n);
return n;
}
EXPORT_SYMBOL(clear_user);
@@ -87,7 +88,7 @@ EXPORT_SYMBOL_GPL(arch_wb_cache_pmem);
long __copy_user_flushcache(void *dst, const void __user *src, unsigned size)
{
unsigned long flushed, dest = (unsigned long) dst;
- long rc = __copy_user_nocache(dst, src, size, 0);
+ long rc = __copy_user_nocache(dst, mask_user_ptr(src), size, 0);

/*
* __copy_user_nocache() uses non-temporal stores for the bulk
--
2.31.1