[PATCH] vsprintf: don't obfuscate NULL and error pointers

From: Ilya Dryomov
Date: Mon Feb 17 2020 - 17:27:55 EST

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@xxxxxxxxx>
lib/vsprintf.c | 7 +++++++
1 file changed, 7 insertions(+)

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);