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

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Date:   Wed, 19 Oct 2022 13:34:31 -0600
From:   Jane Chu <jane.chu@...cle.com>
To:     pmladek@...e.com, rostedt@...dmis.org, senozhatsky@...omium.org,
        andriy.shevchenko@...ux.intel.com, linux@...musvillemoes.dk,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Cc:     wangkefeng.wang@...wei.com, konrad.wilk@...cle.com,
        haakon.bugge@...cle.com, john.haxby@...cle.com, jane.chu@...cle.com
Subject: [PATCH v3 1/1] vsprintf: protect kernel from panic due to non-canonical pointer dereference

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 addes a check on non-canonical string pointer by
kern_addr_valid() and display "(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@...cle.com>
---
 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