x86/asm/memcpy_mcsafe: Add write-protection-fault handling

djbw · Ingo Molnar · commit 12c89130a56a · 2018-05-15T08:32:42.000+02:00
In preparation for using memcpy_mcsafe() to handle user copies it needs to be to handle write-protection faults while writing user pages. Add MMU-fault handlers alongside the machine-check exception handlers. Note that the machine check fault exception handling makes assumptions about source buffer alignment and poison alignment. In the write fault case, given the destination buffer is arbitrarily aligned, it needs a separate / additional fault handling approach. The mcsafe_handle_tail() helper is reused. The @limit argument is set to @len since there is no safety concern about retriggering an MMU fault, and this simplifies the assembly. Co-developed-by: Tony Luck <tony.luck@intel.com> Reported-by: Mika Penttilä <mika.penttila@nextfour.com> Signed-off-by: Dan Williams <dan.j.williams@intel.com> Cc: Al Viro <viro@zeniv.linux.org.uk> Cc: Andrew Morton <akpm@linux-foundation.org> Cc: Andy Lutomirski <luto@amacapital.net> Cc: Borislav Petkov <bp@alien8.de> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: Tony Luck <tony.luck@intel.com> Cc: hch@lst.de Cc: linux-fsdevel@vger.kernel.org Cc: linux-nvdimm@lists.01.org Link: http://lkml.kernel.org/r/152539238635.31796.14056325365122961778.stgit@dwillia2-desk3.amr.corp.intel.com Signed-off-by: Ingo Molnar <mingo@kernel.org>
diff --git a/arch/x86/include/asm/uaccess_64.h b/arch/x86/include/asm/uaccess_64.h
@@ -194,4 +194,7 @@ __copy_from_user_flushcache(void *dst, const void __user *src, unsigned size)
 unsigned long
 copy_user_handle_tail(char *to, char *from, unsigned len);
 
+unsigned long
+mcsafe_handle_tail(char *to, char *from, unsigned len);
+
 #endif /* _ASM_X86_UACCESS_64_H */
diff --git a/arch/x86/lib/memcpy_64.S b/arch/x86/lib/memcpy_64.S
@@ -265,9 +265,23 @@ EXPORT_SYMBOL_GPL(__memcpy_mcsafe)
 	mov	%ecx, %eax
 	ret
 
+	/*
+	 * For write fault handling, given the destination is unaligned,
+	 * we handle faults on multi-byte writes with a byte-by-byte
+	 * copy up to the write-protected page.
+	 */
+.E_write_words:
+	shll	$3, %ecx
+	addl	%edx, %ecx
+	movl	%ecx, %edx
+	jmp mcsafe_handle_tail
+
 	.previous
 
 	_ASM_EXTABLE_FAULT(.L_read_leading_bytes, .E_leading_bytes)
 	_ASM_EXTABLE_FAULT(.L_read_words, .E_read_words)
 	_ASM_EXTABLE_FAULT(.L_read_trailing_bytes, .E_trailing_bytes)
+	_ASM_EXTABLE(.L_write_leading_bytes, .E_leading_bytes)
+	_ASM_EXTABLE(.L_write_words, .E_write_words)
+	_ASM_EXTABLE(.L_write_trailing_bytes, .E_trailing_bytes)
 #endif
diff --git a/arch/x86/lib/usercopy_64.c b/arch/x86/lib/usercopy_64.c
@@ -75,6 +75,27 @@ copy_user_handle_tail(char *to, char *from, unsigned len)
 	return len;
 }
 
+/*
+ * Similar to copy_user_handle_tail, probe for the write fault point,
+ * but reuse __memcpy_mcsafe in case a new read error is encountered.
+ * clac() is handled in _copy_to_iter_mcsafe().
+ */
+__visible unsigned long
+mcsafe_handle_tail(char *to, char *from, unsigned len)
+{
+	for (; len; --len, to++, from++) {
+		/*
+		 * Call the assembly routine back directly since
+		 * memcpy_mcsafe() may silently fallback to memcpy.
+		 */
+		unsigned long rem = __memcpy_mcsafe(to, from, 1);
+
+		if (rem)
+			break;
+	}
+	return len;
+}
+
 #ifdef CONFIG_ARCH_HAS_UACCESS_FLUSHCACHE
 /**
  * clean_cache_range - write back a cache range with CLWB
