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Date: Tue, 19 May 2020 15:26:21 +0300
From: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To: Ilya Dryomov <idryomov@...il.com>
Cc: Petr Mladek <pmladek@...e.com>,
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>,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] vsprintf: don't obfuscate NULL and error pointers
On Tue, May 19, 2020 at 01:26:57PM +0200, Ilya Dryomov wrote:
> 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>
FWIW,
Reviewed-by: Andy Shevchenko <andriy.shevchenko@...ux.intel.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
>
--
With Best Regards,
Andy Shevchenko
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