arm64: Avoid premature usercopy failure

Al reminds us that the usercopy API must only return complete failure
if absolutely nothing could be copied. Currently, if userspace does
something silly like giving us an unaligned pointer to Device memory,
or a size which overruns MTE tag bounds, we may fail to honour that
requirement when faulting on a multi-byte access even though a smaller
access could have succeeded.

Add a mitigation to the fixup routines to fall back to a single-byte
copy if we faulted on a larger access before anything has been written
to the destination, to guarantee making *some* forward progress. We
needn't be too concerned about the overall performance since this should
only occur when callers are doing something a bit dodgy in the first
place. Particularly broken userspace might still be able to trick
generic_perform_write() into an infinite loop by targeting write() at
an mmap() of some read-only device register where the fault-in load
succeeds but any store synchronously aborts such that copy_to_user() is
genuinely unable to make progress, but, well, don't do that...

CC: stable@vger.kernel.org
Reported-by: Chen Huang <chenhuang5@huawei.com>
Suggested-by: Al Viro <viro@zeniv.linux.org.uk>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Link: https://lore.kernel.org/r/dc03d5c675731a1f24a62417dba5429ad744234e.1626098433.git.robin.murphy@arm.com
Signed-off-by: Will Deacon <will@kernel.org>

arch/arm64/lib/copy_from_user.S

@@ -29,32 +29,34 @@
 	.endm
 
 	.macro ldrh1 reg, ptr, val
-	user_ldst 9998f, ldtrh, \reg, \ptr, \val
+	user_ldst 9997f, ldtrh, \reg, \ptr, \val
 	.endm
 
 	.macro strh1 reg, ptr, val
 	strh \reg, [\ptr], \val
 	.endm
 
 	.macro ldr1 reg, ptr, val
-	user_ldst 9998f, ldtr, \reg, \ptr, \val
+	user_ldst 9997f, ldtr, \reg, \ptr, \val
 	.endm
 
 	.macro str1 reg, ptr, val
 	str \reg, [\ptr], \val
 	.endm
 
 	.macro ldp1 reg1, reg2, ptr, val
-	user_ldp 9998f, \reg1, \reg2, \ptr, \val
+	user_ldp 9997f, \reg1, \reg2, \ptr, \val
 	.endm
 
 	.macro stp1 reg1, reg2, ptr, val
 	stp \reg1, \reg2, [\ptr], \val
 	.endm
 
 end	.req	x5
+srcin	.req	x15
 SYM_FUNC_START(__arch_copy_from_user)
 	add	end, x0, x2
+	mov	srcin, x1
 #include "copy_template.S"
 	mov	x0, #0				// Nothing to copy
 	ret
@@ -63,6 +65,11 @@ EXPORT_SYMBOL(__arch_copy_from_user)
 
 	.section .fixup,"ax"
 	.align	2
+9997:	cmp	dst, dstin
+	b.ne	9998f
+	// Before being absolutely sure we couldn't copy anything, try harder
+USER(9998f, ldtrb tmp1w, [srcin])
+	strb	tmp1w, [dst], #1
 9998:	sub	x0, end, dst			// bytes not copied
 	ret
 	.previous

arch/arm64/lib/copy_in_user.S

@@ -30,33 +30,34 @@
 	.endm
 
 	.macro ldrh1 reg, ptr, val
-	user_ldst 9998f, ldtrh, \reg, \ptr, \val
+	user_ldst 9997f, ldtrh, \reg, \ptr, \val
 	.endm
 
 	.macro strh1 reg, ptr, val
-	user_ldst 9998f, sttrh, \reg, \ptr, \val
+	user_ldst 9997f, sttrh, \reg, \ptr, \val
 	.endm
 
 	.macro ldr1 reg, ptr, val
-	user_ldst 9998f, ldtr, \reg, \ptr, \val
+	user_ldst 9997f, ldtr, \reg, \ptr, \val
 	.endm
 
 	.macro str1 reg, ptr, val
-	user_ldst 9998f, sttr, \reg, \ptr, \val
+	user_ldst 9997f, sttr, \reg, \ptr, \val
 	.endm
 
 	.macro ldp1 reg1, reg2, ptr, val
-	user_ldp 9998f, \reg1, \reg2, \ptr, \val
+	user_ldp 9997f, \reg1, \reg2, \ptr, \val
 	.endm
 
 	.macro stp1 reg1, reg2, ptr, val
-	user_stp 9998f, \reg1, \reg2, \ptr, \val
+	user_stp 9997f, \reg1, \reg2, \ptr, \val
 	.endm
 
 end	.req	x5
-
+srcin	.req	x15
 SYM_FUNC_START(__arch_copy_in_user)
 	add	end, x0, x2
+	mov	srcin, x1
 #include "copy_template.S"
 	mov	x0, #0
 	ret
@@ -65,6 +66,12 @@ EXPORT_SYMBOL(__arch_copy_in_user)
 
 	.section .fixup,"ax"
 	.align	2
+9997:	cmp	dst, dstin
+	b.ne	9998f
+	// Before being absolutely sure we couldn't copy anything, try harder
+USER(9998f, ldtrb tmp1w, [srcin])
+USER(9998f, sttrb tmp1w, [dst])
+	add	dst, dst, #1
 9998:	sub	x0, end, dst			// bytes not copied
 	ret
 	.previous

arch/arm64/lib/copy_to_user.S

@@ -32,28 +32,30 @@
 	.endm
 
 	.macro strh1 reg, ptr, val
-	user_ldst 9998f, sttrh, \reg, \ptr, \val
+	user_ldst 9997f, sttrh, \reg, \ptr, \val
 	.endm
 
 	.macro ldr1 reg, ptr, val
 	ldr \reg, [\ptr], \val
 	.endm
 
 	.macro str1 reg, ptr, val
-	user_ldst 9998f, sttr, \reg, \ptr, \val
+	user_ldst 9997f, sttr, \reg, \ptr, \val
 	.endm
 
 	.macro ldp1 reg1, reg2, ptr, val
 	ldp \reg1, \reg2, [\ptr], \val
 	.endm
 
 	.macro stp1 reg1, reg2, ptr, val
-	user_stp 9998f, \reg1, \reg2, \ptr, \val
+	user_stp 9997f, \reg1, \reg2, \ptr, \val
 	.endm
 
 end	.req	x5
+srcin	.req	x15
 SYM_FUNC_START(__arch_copy_to_user)
 	add	end, x0, x2
+	mov	srcin, x1
 #include "copy_template.S"
 	mov	x0, #0
 	ret
@@ -62,6 +64,12 @@ EXPORT_SYMBOL(__arch_copy_to_user)
 
 	.section .fixup,"ax"
 	.align	2
+9997:	cmp	dst, dstin
+	b.ne	9998f
+	// Before being absolutely sure we couldn't copy anything, try harder
+	ldrb	tmp1w, [srcin]
+USER(9998f, sttrb tmp1w, [dst])
+	add	dst, dst, #1
 9998:	sub	x0, end, dst			// bytes not copied
 	ret
 	.previous