Date: Mon, 14 Oct 2013 11:17:06 +0100 From: Djalal Harouni <tixxdz@...ndz.org> To: Ryan Mallon <rmallon@...il.com> Cc: "Eric W. Biederman" <ebiederm@...ssion.com>, Joe Perches <joe@...ches.com>, Andrew Morton <akpm@...ux-foundation.org>, eldad@...refinery.com, Jiri Kosina <jkosina@...e.cz>, jgunthorpe@...idianresearch.com, Dan Rosenberg <dan.j.rosenberg@...il.com>, Kees Cook <keescook@...omium.org>, Alexander Viro <viro@...iv.linux.org.uk>, George Spelvin <linux@...izon.com>, "kernel-hardening@...ts.openwall.com" <kernel-hardening@...ts.openwall.com>, "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org> Subject: Re: [PATCH v3a] vsprintf: Check real user/group id for %pK On Fri, Oct 11, 2013 at 02:19:14PM +1100, Ryan Mallon wrote: > On 11/10/13 13:20, Eric W. Biederman wrote: > > Joe Perches <joe@...ches.com> writes: > > > >> Some setuid binaries will allow reading of files which have read > >> permission by the real user id. This is problematic with files which > >> use %pK because the file access permission is checked at open() time, > >> but the kptr_restrict setting is checked at read() time. If a setuid > >> binary opens a %pK file as an unprivileged user, and then elevates > >> permissions before reading the file, then kernel pointer values may be > >> leaked. > >> > >> This happens for example with the setuid pppd application on Ubuntu > >> 12.04: > >> > >> $ head -1 /proc/kallsyms > >> 00000000 T startup_32 > >> > >> $ pppd file /proc/kallsyms > >> pppd: In file /proc/kallsyms: unrecognized option 'c1000000' > >> > >> This will only leak the pointer value from the first line, but other > >> setuid binaries may leak more information. > >> > >> Fix this by adding a check that in addition to the current process > >> having CAP_SYSLOG, that effective user and group ids are equal to the > >> real ids. If a setuid binary reads the contents of a file which uses > >> %pK then the pointer values will be printed as NULL if the real user > >> is unprivileged. > >> > >> Update the sysctl documentation to reflect the changes, and also > >> correct the documentation to state the kptr_restrict=0 is the default. > > > > Sigh. This is all wrong. The only correct thing to test is > > file->f_cred. Aka the capabilities of the program that opened the > > file. > > > > Which means that the interface to %pK in the case of kptr_restrict is > > broken as it has no way to be passed the information it needs to make > > a sensible decision. > > Would it make sense to add a struct file * to struct seq_file and set > that in seq_open? Then the capability check can be done against seq->file. For the "add a struct file * to struct seq_file" and set it during seq_open(), It was proposed by Linus, but Al Viro didn't like it: https://lkml.org/lkml/2013/9/25/765 I'm not sure if this will work for you: you can make seq_file->private cache some data, by calling single_open()... at ->open(), later check it during read()... As noted by Eric, I'll also go for the capability check at ->open(), if it does not break some userspace. BTW the CAP_SYSLOG check should do the job Checks during read() are not sufficient, since the design allows passing file descriptors and dup() stdin/stdout of suid-execve. IMO: unprivileged code should not get that file descriptor, so ->open() should fail. If this will break userspace then allow open() and cache result for read() Can you emulate the behaviour of kptr_restrict=1 ? If so: 1) perform check during open() and cache data 2) during read() check kptr_restrict==1 check the cached value and if opener had CAP_SYSLOG if so: print something like this: 00000000 T startup_32 All this without modifying vsprintf, I mean just do the checks outside vsprintf() inside your ->read() Just my 2 cents Please cc:me, Thanks -- Djalal Harouni http://opendz.org
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