linux-kernel - [PATCH v3] vsprintf: don't obfuscate NULL and error pointers

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Message-Id: <20200519112657.17098-1-idryomov@gmail.com>
Date:   Tue, 19 May 2020 13:26:57 +0200
From:   Ilya Dryomov <idryomov@...il.com>
To:     Petr Mladek <pmladek@...e.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        Kees Cook <keescook@...omium.org>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        "Tobin C . Harding" <me@...in.cc>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        linux-kernel@...r.kernel.org
Subject: [PATCH v3] vsprintf: don't obfuscate NULL and error pointers

I don't see what security concern is addressed by obfuscating NULL
and IS_ERR() error pointers, printed with %p/%pK.  Given the number
of sites where %p is used (over 10000) and the fact that NULL pointers
aren't uncommon, it probably wouldn't take long for an attacker to
find the hash that corresponds to 0.  Although harder, the same goes
for most common error values, such as -1, -2, -11, -14, etc.

The NULL part actually fixes a regression: NULL pointers weren't
obfuscated until commit 3e5903eb9cff ("vsprintf: Prevent crash when
dereferencing invalid pointers") which went into 5.2.  I'm tacking
the IS_ERR() part on here because error pointers won't leak kernel
addresses and printing them as pointers shouldn't be any different
from e.g. %d with PTR_ERR_OR_ZERO().  Obfuscating them just makes
debugging based on existing pr_debug and friends excruciating.

Note that the "always print 0's for %pK when kptr_restrict == 2"
behaviour which goes way back is left as is.

Example output with the patch applied:

                            ptr         error-ptr              NULL
%p:            0000000001f8cc5b  fffffffffffffff2  0000000000000000
%pK, kptr = 0: 0000000001f8cc5b  fffffffffffffff2  0000000000000000
%px:           ffff888048c04020  fffffffffffffff2  0000000000000000
%pK, kptr = 1: ffff888048c04020  fffffffffffffff2  0000000000000000
%pK, kptr = 2: 0000000000000000  0000000000000000  0000000000000000

Fixes: 3e5903eb9cff ("vsprintf: Prevent crash when dereferencing invalid pointers")
Signed-off-by: Ilya Dryomov <idryomov@...il.com>
Reviewed-by: Petr Mladek <pmladek@...e.com>
Reviewed-by: Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Acked-by: Steven Rostedt (VMware) <rostedt@...dmis.org>
Acked-by: Linus Torvalds <torvalds@...ux-foundation.org>
---
 lib/test_printf.c | 19 ++++++++++++++++++-
 lib/vsprintf.c    |  7 +++++++
 2 files changed, 25 insertions(+), 1 deletion(-)

Hi Petr,

This just came up again, please consider sending this to Linus
for 5.7.

Prior discussion was split in three threads and revolved around the
vision for how lib/test_printf.c should be structured between Rasmus
and yourself.  The fix itself wasn't disputed and has several acks.

If you want to restructure the test suite before adding any new
test cases, v1 doesn't have them, but I'm reposting with test cases
because I think it's best to add them right away to prevent further
regressions.

v3:
- don't use EAGAIN macro in error_pointer() test case as the
  actual error code varies between architectures

v2:
- fix null_pointer() test case (it didn't catch the original
  regression because test_hashed() doesn't really test much)
  and add error_pointer() test case

diff --git a/lib/test_printf.c b/lib/test_printf.c
index 2d9f520d2f27..6b1622f4d7c2 100644
--- a/lib/test_printf.c
+++ b/lib/test_printf.c
@@ -214,6 +214,7 @@ test_string(void)
 #define PTR_STR "ffff0123456789ab"
 #define PTR_VAL_NO_CRNG "(____ptrval____)"
 #define ZEROS "00000000"	/* hex 32 zero bits */
+#define ONES "ffffffff"		/* hex 32 one bits */
 
 static int __init
 plain_format(void)
@@ -245,6 +246,7 @@ plain_format(void)
 #define PTR_STR "456789ab"
 #define PTR_VAL_NO_CRNG "(ptrval)"
 #define ZEROS ""
+#define ONES ""
 
 static int __init
 plain_format(void)
@@ -330,14 +332,28 @@ test_hashed(const char *fmt, const void *p)
 	test(buf, fmt, p);
 }
 
+/*
+ * NULL pointers aren't hashed.
+ */
 static void __init
 null_pointer(void)
 {
-	test_hashed("%p", NULL);
+	test(ZEROS "00000000", "%p", NULL);
 	test(ZEROS "00000000", "%px", NULL);
 	test("(null)", "%pE", NULL);
 }
 
+/*
+ * Error pointers aren't hashed.
+ */
+static void __init
+error_pointer(void)
+{
+	test(ONES "fffffff5", "%p", ERR_PTR(-11));
+	test(ONES "fffffff5", "%px", ERR_PTR(-11));
+	test("(efault)", "%pE", ERR_PTR(-11));
+}
+
 #define PTR_INVALID ((void *)0x000000ab)
 
 static void __init
@@ -649,6 +665,7 @@ test_pointer(void)
 {
 	plain();
 	null_pointer();
+	error_pointer();
 	invalid_pointer();
 	symbol_ptr();
 	kernel_ptr();
diff --git a/lib/vsprintf.c b/lib/vsprintf.c
index 7c488a1ce318..f0f0522cd5a7 100644
--- a/lib/vsprintf.c
+++ b/lib/vsprintf.c
@@ -794,6 +794,13 @@ static char *ptr_to_id(char *buf, char *end, const void *ptr,
 	unsigned long hashval;
 	int ret;
 
+	/*
+	 * Print the real pointer value for NULL and error pointers,
+	 * as they are not actual addresses.
+	 */
+	if (IS_ERR_OR_NULL(ptr))
+		return pointer_string(buf, end, ptr, spec);
+
 	/* When debugging early boot use non-cryptographically secure hash. */
 	if (unlikely(debug_boot_weak_hash)) {
 		hashval = hash_long((unsigned long)ptr, 32);
-- 
2.19.2