Commit 7bd57fbc authored May 19, 2020 by Ilya Dryomov Committed by Linus Torvalds May 19, 2020

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 3e5903eb ("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: 3e5903eb ("vsprintf: Prevent crash when dereferencing invalid pointers")
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Reviewed-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

parent 642b151f

lib/test_printf.c

+18 −1

Original line number	Diff line number	Diff line
		@@ -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();

lib/vsprintf.c

+7 −0

Original line number	Diff line number	Diff line
		@@ -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);

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