Messages in this thread | | | From | Ilya Dryomov <> | Date | Wed, 19 Feb 2020 19:07:10 +0100 | Subject | Re: [PATCH v2] vsprintf: don't obfuscate NULL and error pointers |
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On Wed, Feb 19, 2020 at 6:37 PM Andy Shevchenko <andy.shevchenko@gmail.com> wrote: > > 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?
Yes. It shows that for %pE an error pointer is still invalid. %pe is tested separately, in errptr(), and the output would have been "-EAGAIN".
> Maybe we may use something else ('%ph'?) for sake of deviation?
If you look at the resulting file, you will see that null_pointer(), error_pointer() and invalid_pointer() exercise the same three variants: %p, %px and %pE.
This is somewhat confusing, but there seems to be some disagreement between Pavel and Rasmus as to how the test suite should be structured and I didn't want to attempt to restructure anything in this patch.
Thanks,
Ilya
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