Date: Fri, 11 Oct 2013 14:19:14 +1100 From: Ryan Mallon <rmallon@...il.com> To: "Eric W. Biederman" <ebiederm@...ssion.com>, Joe Perches <joe@...ches.com> CC: 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 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. ~Ryan
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