[PATCH v3 1/1] vsprintf: protect kernel from panic due to non-canonical pointer dereference

[Jane Chu]

Having stepped on a local kernel bug where reading sysfs has led to
out-of-bound pointer dereference by vsprintf() which led to GPF panic.
And the reason for GPF is that the OOB pointer was turned to a
non-canonical address such as 0x7665645f63616465.

vsprintf() already has this line of defense
	if ((unsigned long)ptr < PAGE_SIZE || IS_ERR_VALUE(ptr))
                return "(efault)";
Since a non-canonical pointer can be detected by kern_addr_valid()
on architectures that present VM holes as well as meaningful
implementation of kern_addr_valid() that detects the non-canonical
addresses, this patch adds a check on non-canonical string pointer by
kern_addr_valid() and "(efault)" to alert user that something
is wrong instead of unecessarily panic the server.

On the other hand, if the non-canonical string pointer is dereferenced
else where in the kernel, by virtue of being non-canonical, a crash
is expected to be immediate.

Signed-off-by: Jane Chu <jane.chu@xxxxxxxxxx>
---
 lib/vsprintf.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/lib/vsprintf.c b/lib/vsprintf.c
index c414a8d9f1ea..b38c12ef1e45 100644
--- a/lib/vsprintf.c
+++ b/lib/vsprintf.c
@@ -698,6 +698,9 @@ static const char *check_pointer_msg(const void *ptr)
 	if ((unsigned long)ptr < PAGE_SIZE || IS_ERR_VALUE(ptr))
 		return "(efault)";
 
+	if (!kern_addr_valid((unsigned long)ptr))
+		return "(efault)";
+
 	return NULL;
 }
 
-- 
2.18.4