Messages in this thread | | | From | Andy Shevchenko <> | Date | Wed, 19 Feb 2020 19:37:29 +0200 | Subject | Re: [PATCH v2] vsprintf: don't obfuscate NULL and error pointers |
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On Wed, Feb 19, 2020 at 7:13 PM Ilya Dryomov <idryomov@gmail.com> 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
...
> +/* > + * 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(-EAGAIN)); > + test(ONES "fffffff5", "%px", ERR_PTR(-EAGAIN));
> + test("(efault)", "%pE", ERR_PTR(-EAGAIN));
Hmm... Is capital E on purpose here? Maybe we may use something else ('%ph'?) for sake of deviation?
> +}
-- With Best Regards, Andy Shevchenko
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