[PATCH v2] x86/uaccess: Use pointer masking to limit uaccess speculation

From: Josh Poimboeuf
Date: Tue Sep 01 2020 - 12:23:21 EST

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

barrier_nospec() is now unused, and can be removed.

Fixes: 4b842e4e25b1 ("x86: get rid of small constant size cases in raw_copy_{to,from}_user()")
Suggested-by: Will Deacon <will@xxxxxxxxxx>
Signed-off-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
---
v2:
- fix 32-bit build

Documentation/admin-guide/hw-vuln/spectre.rst | 6 ++--
arch/x86/include/asm/barrier.h | 3 --
arch/x86/include/asm/checksum_32.h | 6 ++--
arch/x86/include/asm/futex.h | 5 +++
arch/x86/include/asm/uaccess.h | 31 ++++++++++++-------
arch/x86/include/asm/uaccess_64.h | 16 +++++-----
arch/x86/lib/csum-wrappers_64.c | 6 ++--
arch/x86/lib/getuser.S | 10 +++---
arch/x86/lib/putuser.S | 8 +++++
arch/x86/lib/usercopy_32.c | 6 ++--
arch/x86/lib/usercopy_64.c | 4 +--
lib/iov_iter.c | 2 +-
12 files changed, 60 insertions(+), 43 deletions(-)

diff --git a/Documentation/admin-guide/hw-vuln/spectre.rst b/Documentation/admin-guide/hw-vuln/spectre.rst
index e05e581af5cf..849c613afb35 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 uaccess_mask_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/barrier.h b/arch/x86/include/asm/barrier.h
index 7f828fe49797..d158ea1fa250 100644
--- a/arch/x86/include/asm/barrier.h
+++ b/arch/x86/include/asm/barrier.h
@@ -48,9 +48,6 @@ static inline unsigned long array_index_mask_nospec(unsigned long index,
/* Override the default implementation from linux/nospec.h. */
#define array_index_mask_nospec array_index_mask_nospec

-/* Prevent speculative execution past this barrier. */
-#define barrier_nospec() alternative("", "lfence", X86_FEATURE_LFENCE_RDTSC)
-
#define dma_rmb() barrier()
#define dma_wmb() barrier()

diff --git a/arch/x86/include/asm/checksum_32.h b/arch/x86/include/asm/checksum_32.h
index 11624c8a9d8d..d2bbea5b0334 100644
--- a/arch/x86/include/asm/checksum_32.h
+++ b/arch/x86/include/asm/checksum_32.h
@@ -56,8 +56,8 @@ static inline __wsum csum_and_copy_from_user(const void __user *src,
*err_ptr = -EFAULT;
return sum;
}
- ret = csum_partial_copy_generic((__force void *)src, dst,
- len, sum, err_ptr, NULL);
+ ret = csum_partial_copy_generic((__force void *)uaccess_mask_ptr(src),
+ dst, len, sum, err_ptr, NULL);
user_access_end();

return ret;
@@ -185,7 +185,7 @@ static inline __wsum csum_and_copy_to_user(const void *src,

might_sleep();
if (user_access_begin(dst, len)) {
- ret = csum_partial_copy_generic(src, (__force void *)dst,
+ ret = csum_partial_copy_generic(src, (__force void *)uaccess_mask_ptr(dst),
len, sum, NULL, err_ptr);
user_access_end();
return ret;
diff --git a/arch/x86/include/asm/futex.h b/arch/x86/include/asm/futex.h
index f9c00110a69a..89e7e37c2b25 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 = uaccess_mask_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 = uaccess_mask_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 ecefaffd15d4..e697b42d0533 100644
--- a/arch/x86/include/asm/uaccess.h
+++ b/arch/x86/include/asm/uaccess.h
@@ -6,6 +6,7 @@
*/
#include <linux/compiler.h>
#include <linux/kasan-checks.h>
+#include <linux/nospec.h>
#include <linux/string.h>
#include <asm/asm.h>
#include <asm/page.h>
@@ -96,6 +97,15 @@ static inline bool pagefault_disabled(void);
likely(!__range_not_ok(addr, size, user_addr_max())); \
})

+/*
+ * Sanitize a uaccess pointer such that it becomes NULL if it's not a valid
+ * user pointer. This blocks speculative dereferences of user-controlled
+ * pointers.
+ */
+#define uaccess_mask_ptr(ptr) \
+ (__typeof__(ptr)) array_index_nospec((__force unsigned long)ptr, user_addr_max())
+
+
/*
* These are the main single-value transfer routines. They automatically
* use the right size if we just have the right pointer type.
@@ -119,11 +129,6 @@ 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
@@ -357,7 +362,7 @@ do { \
__label__ __pu_label; \
int __pu_err = -EFAULT; \
__typeof__(*(ptr)) __pu_val = (x); \
- __typeof__(ptr) __pu_ptr = (ptr); \
+ __typeof__(ptr) __pu_ptr = uaccess_mask_ptr(ptr); \
__typeof__(size) __pu_size = (size); \
__uaccess_begin(); \
__put_user_size(__pu_val, __pu_ptr, __pu_size, __pu_label); \
@@ -371,9 +376,9 @@ __pu_label: \
({ \
int __gu_err; \
__inttype(*(ptr)) __gu_val; \
- __typeof__(ptr) __gu_ptr = (ptr); \
+ __typeof__(ptr) __gu_ptr = uaccess_mask_ptr(ptr); \
__typeof__(size) __gu_size = (size); \
- __uaccess_begin_nospec(); \
+ __uaccess_begin(); \
__get_user_size(__gu_val, __gu_ptr, __gu_size, __gu_err); \
__uaccess_end(); \
(x) = (__force __typeof__(*(ptr)))__gu_val; \
@@ -482,7 +487,7 @@ 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)
@@ -491,14 +496,16 @@ 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), uaccess_mask_ptr(ptr), \
+ sizeof(*(ptr)), label)

#define unsafe_get_user(x, ptr, err_label) \
do { \
int __gu_err; \
__inttype(*(ptr)) __gu_val; \
- __get_user_size(__gu_val, (ptr), sizeof(*(ptr)), __gu_err); \
+ __get_user_size(__gu_val, uaccess_mask_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 bc10e3dc64fe..ef859e1ae956 100644
--- a/arch/x86/include/asm/uaccess_64.h
+++ b/arch/x86/include/asm/uaccess_64.h
@@ -47,7 +47,7 @@ copy_user_generic(void *to, const void *from, unsigned len)
}

static __always_inline __must_check unsigned long
-copy_to_user_mcsafe(void *to, const void *from, unsigned len)
+copy_to_user_mcsafe(void __user *to, const void *from, size_t len)
{
unsigned long ret;

@@ -57,7 +57,7 @@ copy_to_user_mcsafe(void *to, const void *from, unsigned len)
* handle exceptions / faults. memcpy_mcsafe() may fall back to
* memcpy() which lacks this handling.
*/
- ret = __memcpy_mcsafe(to, from, len);
+ ret = __memcpy_mcsafe((__force void *)uaccess_mask_ptr(to), from, len);
__uaccess_end();
return ret;
}
@@ -65,20 +65,20 @@ copy_to_user_mcsafe(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 *)uaccess_mask_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 *)uaccess_mask_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 *)uaccess_mask_ptr(dst),
+ (__force void *)uaccess_mask_ptr(src), size);
}

extern long __copy_user_nocache(void *dst, const void __user *src,
@@ -93,14 +93,14 @@ __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, uaccess_mask_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, uaccess_mask_ptr(src), size);
}

unsigned long
diff --git a/arch/x86/lib/csum-wrappers_64.c b/arch/x86/lib/csum-wrappers_64.c
index ee63d7576fd2..d677b91a7e84 100644
--- a/arch/x86/lib/csum-wrappers_64.c
+++ b/arch/x86/lib/csum-wrappers_64.c
@@ -52,8 +52,8 @@ csum_and_copy_from_user(const void __user *src, void *dst,
len -= 2;
}
}
- isum = csum_partial_copy_generic((__force const void *)src,
- dst, len, isum, errp, NULL);
+ isum = csum_partial_copy_generic((__force const void *)uaccess_mask_ptr(src),
+ dst, len, isum, errp, NULL);
user_access_end();
if (unlikely(*errp))
goto out_err;
@@ -108,7 +108,7 @@ csum_and_copy_to_user(const void *src, void __user *dst,
}

*errp = 0;
- ret = csum_partial_copy_generic(src, (void __force *)dst,
+ ret = csum_partial_copy_generic(src, (void __force *)uaccess_mask_ptr(dst),
len, isum, NULL, errp);
user_access_end();
return ret;
diff --git a/arch/x86/lib/getuser.S b/arch/x86/lib/getuser.S
index c8a85b512796..90ae5be8af23 100644
--- a/arch/x86/lib/getuser.S
+++ b/arch/x86/lib/getuser.S
@@ -40,7 +40,7 @@ SYM_FUNC_START(__get_user_1)
mov PER_CPU_VAR(current_task), %_ASM_DX
cmp TASK_addr_limit(%_ASM_DX),%_ASM_AX
jae bad_get_user
- sbb %_ASM_DX, %_ASM_DX /* array_index_mask_nospec() */
+ sbb %_ASM_DX, %_ASM_DX /* uaccess_mask_ptr() */
and %_ASM_DX, %_ASM_AX
ASM_STAC
1: movzbl (%_ASM_AX),%edx
@@ -56,7 +56,7 @@ SYM_FUNC_START(__get_user_2)
mov PER_CPU_VAR(current_task), %_ASM_DX
cmp TASK_addr_limit(%_ASM_DX),%_ASM_AX
jae bad_get_user
- sbb %_ASM_DX, %_ASM_DX /* array_index_mask_nospec() */
+ sbb %_ASM_DX, %_ASM_DX /* uaccess_mask_ptr() */
and %_ASM_DX, %_ASM_AX
ASM_STAC
2: movzwl -1(%_ASM_AX),%edx
@@ -72,7 +72,7 @@ SYM_FUNC_START(__get_user_4)
mov PER_CPU_VAR(current_task), %_ASM_DX
cmp TASK_addr_limit(%_ASM_DX),%_ASM_AX
jae bad_get_user
- sbb %_ASM_DX, %_ASM_DX /* array_index_mask_nospec() */
+ sbb %_ASM_DX, %_ASM_DX /* uaccess_mask_ptr() */
and %_ASM_DX, %_ASM_AX
ASM_STAC
3: movl -3(%_ASM_AX),%edx
@@ -89,7 +89,7 @@ SYM_FUNC_START(__get_user_8)
mov PER_CPU_VAR(current_task), %_ASM_DX
cmp TASK_addr_limit(%_ASM_DX),%_ASM_AX
jae bad_get_user
- sbb %_ASM_DX, %_ASM_DX /* array_index_mask_nospec() */
+ sbb %_ASM_DX, %_ASM_DX /* uaccess_mask_ptr() */
and %_ASM_DX, %_ASM_AX
ASM_STAC
4: movq -7(%_ASM_AX),%rdx
@@ -102,7 +102,7 @@ SYM_FUNC_START(__get_user_8)
mov PER_CPU_VAR(current_task), %_ASM_DX
cmp TASK_addr_limit(%_ASM_DX),%_ASM_AX
jae bad_get_user_8
- sbb %_ASM_DX, %_ASM_DX /* array_index_mask_nospec() */
+ sbb %_ASM_DX, %_ASM_DX /* uaccess_mask_ptr() */
and %_ASM_DX, %_ASM_AX
ASM_STAC
4: movl -7(%_ASM_AX),%edx
diff --git a/arch/x86/lib/putuser.S b/arch/x86/lib/putuser.S
index 7c7c92db8497..a4384b0ce013 100644
--- a/arch/x86/lib/putuser.S
+++ b/arch/x86/lib/putuser.S
@@ -38,6 +38,8 @@ SYM_FUNC_START(__put_user_1)
ENTER
cmp TASK_addr_limit(%_ASM_BX),%_ASM_CX
jae .Lbad_put_user
+ sbb %_ASM_BX, %_ASM_BX /* uaccess_mask_ptr() */
+ and %_ASM_BX, %_ASM_CX
ASM_STAC
1: movb %al,(%_ASM_CX)
xor %eax,%eax
@@ -52,6 +54,8 @@ SYM_FUNC_START(__put_user_2)
sub $1,%_ASM_BX
cmp %_ASM_BX,%_ASM_CX
jae .Lbad_put_user
+ sbb %_ASM_BX, %_ASM_BX /* uaccess_mask_ptr() */
+ and %_ASM_BX, %_ASM_CX
ASM_STAC
2: movw %ax,(%_ASM_CX)
xor %eax,%eax
@@ -66,6 +70,8 @@ SYM_FUNC_START(__put_user_4)
sub $3,%_ASM_BX
cmp %_ASM_BX,%_ASM_CX
jae .Lbad_put_user
+ sbb %_ASM_BX, %_ASM_BX /* uaccess_mask_ptr() */
+ and %_ASM_BX, %_ASM_CX
ASM_STAC
3: movl %eax,(%_ASM_CX)
xor %eax,%eax
@@ -80,6 +86,8 @@ SYM_FUNC_START(__put_user_8)
sub $7,%_ASM_BX
cmp %_ASM_BX,%_ASM_CX
jae .Lbad_put_user
+ sbb %_ASM_BX, %_ASM_BX /* uaccess_mask_ptr() */
+ and %_ASM_BX, %_ASM_CX
ASM_STAC
4: mov %_ASM_AX,(%_ASM_CX)
#ifdef CONFIG_X86_32
diff --git a/arch/x86/lib/usercopy_32.c b/arch/x86/lib/usercopy_32.c
index 7d290777246d..185ae8f56477 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(uaccess_mask_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 b0dfac3d3df7..abe5d2d1c49a 100644
--- a/arch/x86/lib/usercopy_64.c
+++ b/arch/x86/lib/usercopy_64.c
@@ -51,7 +51,7 @@ 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(uaccess_mask_ptr(to), n);
return n;
}
EXPORT_SYMBOL(clear_user);
@@ -108,7 +108,7 @@ 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, uaccess_mask_ptr(src), size, 0);

/*
* __copy_user_nocache() uses non-temporal stores for the bulk
diff --git a/lib/iov_iter.c b/lib/iov_iter.c
index 5e40786c8f12..175cc8320220 100644
--- a/lib/iov_iter.c
+++ b/lib/iov_iter.c
@@ -642,7 +642,7 @@ static int copyout_mcsafe(void __user *to, const void *from, size_t n)
{
if (access_ok(to, n)) {
instrument_copy_to_user(to, from, n);
- n = copy_to_user_mcsafe((__force void *) to, from, n);
+ n = copy_to_user_mcsafe(to, from, n);
}
return n;
}
--
2.25.4