Merge branch 'x86-uaccess-cleanup': x86 uaccess header cleanups

Merge my x86 uaccess updates branch.

The LAM ("Linear Address Masking") updates in this release made me
unhappy about how "access_ok()" was done, and it actually turned out to
have a couple of small bugs in it too.  This is my cleanup of the code:

 - use the sign bit of the __user pointer rather than masking the
   address and checking it against the TASK_SIZE range.

   We already did this part for the get/put_user() side, but
   'access_ok()' did the naïve "mask and range check" thing, which not
   only generates nasty code, but also ended up meaning that __access_ok
   itself didn't do a good job, and so copy_from_user_nmi() didn't get
   the check right.

 - move all the code that is 64-bit only into the 64-bit version of the
   header file, so that we don't unnecessarily pollute the shared x86
   code and make it look like LAM might work in 32-bit too.

 - fix a bug in the address masking (that doesn't end up mattering: in
   this case the fix was to just remove the buggy code entirely).

 - a couple of trivial cleanups and added commentary about the
   access_ok() rules.

* x86-uaccess-cleanup:
  x86-64: mm: clarify the 'positive addresses' user address rules
  x86: mm: remove 'sign' games from LAM untagged_addr*() macros
  x86: uaccess: move 32-bit and 64-bit parts into proper <asm/uaccess_N.h> header
  x86: mm: remove architecture-specific 'access_ok()' define
  x86-64: make access_ok() independent of LAM

4 files changed, 122 insertions, 94 deletions

diff --git a/arch/x86/include/asm/uaccess.h b/arch/x86/include/asm/uaccess.h
index 457e814712af..8bae40a66282 100644
--- a/arch/x86/include/asm/uaccess.h
+++ b/arch/x86/include/asm/uaccess.h
@@ -16,88 +16,12 @@
 #include <asm/extable.h>
 #include <asm/tlbflush.h>
 
-#ifdef CONFIG_DEBUG_ATOMIC_SLEEP
-static inline bool pagefault_disabled(void);
-# define WARN_ON_IN_IRQ()	\
-	WARN_ON_ONCE(!in_task() && !pagefault_disabled())
-#else
-# define WARN_ON_IN_IRQ()
-#endif
-
-#ifdef CONFIG_ADDRESS_MASKING
-/*
- * Mask out tag bits from the address.
- *
- * Magic with the 'sign' allows to untag userspace pointer without any branches
- * while leaving kernel addresses intact.
- */
-static inline unsigned long __untagged_addr(unsigned long addr)
-{
-	long sign;
-
-	/*
-	 * Refer tlbstate_untag_mask directly to avoid RIP-relative relocation
-	 * in alternative instructions. The relocation gets wrong when gets
-	 * copied to the target place.
-	 */
-	asm (ALTERNATIVE("",
-			 "sar $63, %[sign]\n\t" /* user_ptr ? 0 : -1UL */
-			 "or %%gs:tlbstate_untag_mask, %[sign]\n\t"
-			 "and %[sign], %[addr]\n\t", X86_FEATURE_LAM)
-	     : [addr] "+r" (addr), [sign] "=r" (sign)
-	     : "m" (tlbstate_untag_mask), "[sign]" (addr));
-
-	return addr;
-}
-
-#define untagged_addr(addr)	({					\
-	unsigned long __addr = (__force unsigned long)(addr);		\
-	(__force __typeof__(addr))__untagged_addr(__addr);		\
-})
-
-static inline unsigned long __untagged_addr_remote(struct mm_struct *mm,
-						   unsigned long addr)
-{
-	long sign = addr >> 63;
-
-	mmap_assert_locked(mm);
-	addr &= (mm)->context.untag_mask | sign;
-
-	return addr;
-}
-
-#define untagged_addr_remote(mm, addr)	({				\
-	unsigned long __addr = (__force unsigned long)(addr);		\
-	(__force __typeof__(addr))__untagged_addr_remote(mm, __addr);	\
-})
-
+#ifdef CONFIG_X86_32
+# include <asm/uaccess_32.h>
 #else
-#define untagged_addr(addr)	(addr)
+# include <asm/uaccess_64.h>
 #endif
 
-/**
- * access_ok - Checks if a user space pointer is valid
- * @addr: User space pointer to start of block to check
- * @size: Size of block to check
- *
- * Context: User context only. This function may sleep if pagefaults are
- *          enabled.
- *
- * Checks if a pointer to a block of memory in user space is valid.
- *
- * Note that, depending on architecture, this function probably just
- * checks that the pointer is in the user space range - after calling
- * this function, memory access functions may still return -EFAULT.
- *
- * Return: true (nonzero) if the memory block may be valid, false (zero)
- * if it is definitely invalid.
- */
-#define access_ok(addr, size)						\
-({									\
-	WARN_ON_IN_IRQ();						\
-	likely(__access_ok(untagged_addr(addr), size));			\
-})
-
 #include <asm-generic/access_ok.h>
 
 extern int __get_user_1(void);
@@ -586,14 +510,6 @@ extern struct movsl_mask {
 
 #define ARCH_HAS_NOCACHE_UACCESS 1
 
-#ifdef CONFIG_X86_32
-unsigned long __must_check clear_user(void __user *mem, unsigned long len);
-unsigned long __must_check __clear_user(void __user *mem, unsigned long len);
-# include <asm/uaccess_32.h>
-#else
-# include <asm/uaccess_64.h>
-#endif
-
 /*
  * The "unsafe" user accesses aren't really "unsafe", but the naming
  * is a big fat warning: you have to not only do the access_ok()
diff --git a/arch/x86/include/asm/uaccess_32.h b/arch/x86/include/asm/uaccess_32.h
index 388a40660c7b..40379a1adbb8 100644
--- a/arch/x86/include/asm/uaccess_32.h
+++ b/arch/x86/include/asm/uaccess_32.h
@@ -33,4 +33,7 @@ __copy_from_user_inatomic_nocache(void *to, const void __user *from,
        return __copy_from_user_ll_nocache_nozero(to, from, n);
 }
 
+unsigned long __must_check clear_user(void __user *mem, unsigned long len);
+unsigned long __must_check __clear_user(void __user *mem, unsigned long len);
+
 #endif /* _ASM_X86_UACCESS_32_H */
diff --git a/arch/x86/include/asm/uaccess_64.h b/arch/x86/include/asm/uaccess_64.h
index c972bd21aa23..81b826d3b753 100644
--- a/arch/x86/include/asm/uaccess_64.h
+++ b/arch/x86/include/asm/uaccess_64.h
@@ -12,6 +12,87 @@
 #include <asm/cpufeatures.h>
 #include <asm/page.h>
 
+#ifdef CONFIG_ADDRESS_MASKING
+/*
+ * Mask out tag bits from the address.
+ */
+static inline unsigned long __untagged_addr(unsigned long addr)
+{
+	/*
+	 * Refer tlbstate_untag_mask directly to avoid RIP-relative relocation
+	 * in alternative instructions. The relocation gets wrong when gets
+	 * copied to the target place.
+	 */
+	asm (ALTERNATIVE("",
+			 "and %%gs:tlbstate_untag_mask, %[addr]\n\t", X86_FEATURE_LAM)
+	     : [addr] "+r" (addr) : "m" (tlbstate_untag_mask));
+
+	return addr;
+}
+
+#define untagged_addr(addr)	({					\
+	unsigned long __addr = (__force unsigned long)(addr);		\
+	(__force __typeof__(addr))__untagged_addr(__addr);		\
+})
+
+static inline unsigned long __untagged_addr_remote(struct mm_struct *mm,
+						   unsigned long addr)
+{
+	mmap_assert_locked(mm);
+	return addr & (mm)->context.untag_mask;
+}
+
+#define untagged_addr_remote(mm, addr)	({				\
+	unsigned long __addr = (__force unsigned long)(addr);		\
+	(__force __typeof__(addr))__untagged_addr_remote(mm, __addr);	\
+})
+
+#endif
+
+/*
+ * The virtual address space space is logically divided into a kernel
+ * half and a user half.  When cast to a signed type, user pointers
+ * are positive and kernel pointers are negative.
+ */
+#define valid_user_address(x) ((long)(x) >= 0)
+
+/*
+ * User pointers can have tag bits on x86-64.  This scheme tolerates
+ * arbitrary values in those bits rather then masking them off.
+ *
+ * Enforce two rules:
+ * 1. 'ptr' must be in the user half of the address space
+ * 2. 'ptr+size' must not overflow into kernel addresses
+ *
+ * Note that addresses around the sign change are not valid addresses,
+ * and will GP-fault even with LAM enabled if the sign bit is set (see
+ * "CR3.LAM_SUP" that can narrow the canonicality check if we ever
+ * enable it, but not remove it entirely).
+ *
+ * So the "overflow into kernel addresses" does not imply some sudden
+ * exact boundary at the sign bit, and we can allow a lot of slop on the
+ * size check.
+ *
+ * In fact, we could probably remove the size check entirely, since
+ * any kernel accesses will be in increasing address order starting
+ * at 'ptr', and even if the end might be in kernel space, we'll
+ * hit the GP faults for non-canonical accesses before we ever get
+ * there.
+ *
+ * That's a separate optimization, for now just handle the small
+ * constant case.
+ */
+static inline bool __access_ok(const void __user *ptr, unsigned long size)
+{
+	if (__builtin_constant_p(size <= PAGE_SIZE) && size <= PAGE_SIZE) {
+		return valid_user_address(ptr);
+	} else {
+		unsigned long sum = size + (unsigned long)ptr;
+		return valid_user_address(sum) && sum >= (unsigned long)ptr;
+	}
+}
+#define __access_ok __access_ok
+
 /*
  * Copy To/From Userspace
  */
@@ -106,7 +187,7 @@ static __always_inline __must_check unsigned long __clear_user(void __user *addr
 
 static __always_inline unsigned long clear_user(void __user *to, unsigned long n)
 {
-	if (access_ok(to, n))
+	if (__access_ok(to, n))
 		return __clear_user(to, n);
 	return n;
 }
diff --git a/arch/x86/mm/extable.c b/arch/x86/mm/extable.c
index 60814e110a54..271dcb2deabc 100644
--- a/arch/x86/mm/extable.c
+++ b/arch/x86/mm/extable.c
@@ -130,10 +130,36 @@ static bool ex_handler_fprestore(const struct exception_table_entry *fixup,
 	return true;
 }
 
+/*
+ * On x86-64, we end up being imprecise with 'access_ok()', and allow
+ * non-canonical user addresses to make the range comparisons simpler,
+ * and to not have to worry about LAM being enabled.
+ *
+ * In fact, we allow up to one page of "slop" at the sign boundary,
+ * which means that we can do access_ok() by just checking the sign
+ * of the pointer for the common case of having a small access size.
+ */
+static bool gp_fault_address_ok(unsigned long fault_address)
+{
+#ifdef CONFIG_X86_64
+	/* Is it in the "user space" part of the non-canonical space? */
+	if (valid_user_address(fault_address))
+		return true;
+
+	/* .. or just above it? */
+	fault_address -= PAGE_SIZE;
+	if (valid_user_address(fault_address))
+		return true;
+#endif
+	return false;
+}
+
 static bool ex_handler_uaccess(const struct exception_table_entry *fixup,
-			       struct pt_regs *regs, int trapnr)
+			       struct pt_regs *regs, int trapnr,
+			       unsigned long fault_address)
 {
-	WARN_ONCE(trapnr == X86_TRAP_GP, "General protection fault in user access. Non-canonical address?");
+	WARN_ONCE(trapnr == X86_TRAP_GP && !gp_fault_address_ok(fault_address),
+		"General protection fault in user access. Non-canonical address?");
 	return ex_handler_default(fixup, regs);
 }
 
@@ -189,10 +215,12 @@ static bool ex_handler_imm_reg(const struct exception_table_entry *fixup,
 }
 
 static bool ex_handler_ucopy_len(const struct exception_table_entry *fixup,
-				  struct pt_regs *regs, int trapnr, int reg, int imm)
+				  struct pt_regs *regs, int trapnr,
+				  unsigned long fault_address,
+				  int reg, int imm)
 {
 	regs->cx = imm * regs->cx + *pt_regs_nr(regs, reg);
-	return ex_handler_uaccess(fixup, regs, trapnr);
+	return ex_handler_uaccess(fixup, regs, trapnr, fault_address);
 }
 
 int ex_get_fixup_type(unsigned long ip)
@@ -238,7 +266,7 @@ int fixup_exception(struct pt_regs *regs, int trapnr, unsigned long error_code,
 	case EX_TYPE_FAULT_MCE_SAFE:
 		return ex_handler_fault(e, regs, trapnr);
 	case EX_TYPE_UACCESS:
-		return ex_handler_uaccess(e, regs, trapnr);
+		return ex_handler_uaccess(e, regs, trapnr, fault_addr);
 	case EX_TYPE_COPY:
 		return ex_handler_copy(e, regs, trapnr);
 	case EX_TYPE_CLEAR_FS:
@@ -269,7 +297,7 @@ int fixup_exception(struct pt_regs *regs, int trapnr, unsigned long error_code,
 	case EX_TYPE_FAULT_SGX:
 		return ex_handler_sgx(e, regs, trapnr);
 	case EX_TYPE_UCOPY_LEN:
-		return ex_handler_ucopy_len(e, regs, trapnr, reg, imm);
+		return ex_handler_ucopy_len(e, regs, trapnr, fault_addr, reg, imm);
 	case EX_TYPE_ZEROPAD:
 		return ex_handler_zeropad(e, regs, fault_addr);
 	}