Date: Tue, 31 Oct 2017 09:45:44 +1100 From: "Tobin C. Harding" <me@...in.cc> To: Kees Cook <keescook@...omium.org> Cc: kernel-hardening@...ts.openwall.com, "Jason A. Donenfeld" <Jason@...c4.com>, Theodore Ts'o <tytso@....edu>, Linus Torvalds <torvalds@...ux-foundation.org>, Paolo Bonzini <pbonzini@...hat.com>, Tycho Andersen <tycho@...ker.com>, "Roberts, William C" <william.c.roberts@...el.com>, Tejun Heo <tj@...nel.org>, Jordan Glover <Golden_Miller83@...tonmail.ch>, Greg KH <gregkh@...uxfoundation.org>, Petr Mladek <pmladek@...e.com>, Joe Perches <joe@...ches.com>, Ian Campbell <ijc@...lion.org.uk>, Sergey Senozhatsky <sergey.senozhatsky@...il.com>, Catalin Marinas <catalin.marinas@....com>, Will Deacon <wilal.deacon@....com>, Steven Rostedt <rostedt@...dmis.org>, Chris Fries <cfries@...gle.com>, Dave Weinstein <olorin@...gle.com>, Daniel Micay <danielmicay@...il.com>, Djalal Harouni <tixxdz@...il.com>, LKML <linux-kernel@...r.kernel.org> Subject: Re: [PATCH V9] printk: hash addresses printed with %p On Mon, Oct 30, 2017 at 03:31:41PM -0700, Kees Cook wrote: > On Sun, Oct 29, 2017 at 3:59 PM, Tobin C. Harding <me@...in.cc> wrote: > > Currently there are many places in the kernel where addresses are being > > printed using an unadorned %p. Kernel pointers should be printed using > > %pK allowing some control via the kptr_restrict sysctl. Exposing addresses > > gives attackers sensitive information about the kernel layout in memory. > > > > We can reduce the attack surface by hashing all addresses printed with > > %p. This will of course break some users, forcing code printing needed > > addresses to be updated. > > > > For what it's worth, usage of unadorned %p can be broken down as > > follows (thanks to Joe Perches). > > > > $ git grep -E '%p[^A-Za-z0-9]' | cut -f1 -d"/" | sort | uniq -c > > 1084 arch > > 20 block > > 10 crypto > > 32 Documentation > > 8121 drivers > > 1221 fs > > 143 include > > 101 kernel > > 69 lib > > 100 mm > > 1510 net > > 40 samples > > 7 scripts > > 11 security > > 166 sound > > 152 tools > > 2 virt > > > > Add function ptr_to_id() to map an address to a 32 bit unique > > identifier. Hash any unadorned usage of specifier %p and any malformed > > specifiers. > > > > Signed-off-by: Tobin C. Harding <me@...in.cc> > > > > --- > > > > It seems we don't have consensus on a couple of things > > > > 1. The size of the hashed address on 64 bit architectures. > > 2. The use of '0x' pre-fix for hashed addresses. > > > > In regards to (1), we are agreed that we only need 32 bits of > > information. There is some questions however that outputting _only_ 32 > > bits may break userland. > > > > In regards to (2), irrespective of the arguments for and against, if > > point 1 is correct and changing the format will break userland then we > > can't add the '0x' suffix for the same reason. > > > > Therefore this patch masks off the first 32 bits, retaining > > only 32 bits of information. We do not add a '0x' suffix. All in all, > > that results in _no_ change to the format of output only the content of > > the output. > > > > The leading 0's also make explicit that we have messed with the address, > > maybe this will save some debugging time by doing so. Although this > > would probably already be obvious since there is no leading 'ffff'. > > > > We hash malformed specifiers also. Malformed specifiers include > > incomplete (e.g %pi) and also non-existent specifiers. checkpatch should > > warn for non-existent specifiers but AFAICT won't warn for incomplete > > specifiers. > > > > Here is the behaviour that this patch implements. > > > > For kpt_restrict==0 > > > > Randomness not ready: > > printed with %p: (pointer value) # NOTE: with padding > > Valid pointer: > > printed with %pK: deadbeefdeadbeef > > printed with %p: 00000000deadbeef > > malformed specifier (eg %i): 00000000deadbeef > > NULL pointer: > > printed with %pK: 0000000000000000 > > printed with %p: (null) # NOTE: with padding > > malformed specifier (eg %i): (null) > > > > For kpt_restrict==2 > > > > Valid pointer: > > printed with %pK: 0000000000000000 > > > > All other output as for kptr_restrict==0 > > > > V9: > > - Drop the initial patch from V8, leaving null pointer handling as is. > > - Print the hashed ID _without_ a '0x' suffix. > > - Mask the first 32 bits of the hashed ID to all zeros on 64 bit > > architectures. > > Oops, I had missed v9. This addresses my concerns. I think the leading > zeros are a good way to identify the "this is clearly not a kernel > address" issue (though the 32-bit folks may remain confused, but we > can fix that later, IMO). Awesome. Yeah this patch (coupled with the leaking_addresses.pl script) is turning out to be a bit 64-bit centric. However, as we plug more leaks in 64-bit kernels hopefully they will be plugged in 32-bit ones too. I can't think of any way to have leaking_addresses.pl grep for 32-bit addresses, especially once/if this patch gets merged. We will not be able to differentiate between hashed addresses and real addresses on 32-bit machines. thanks, Tobin.
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