Ryan Mallon <rmal...@gmail.com> writes: > On 11/10/13 13:20, Eric W. Biederman wrote: >> Joe Perches <j...@perches.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.
It would make most sense to do the capability check at open time, and cache the result. Doing it generically so that seq_printf could still use %pK doesn't sound wrong, but it does sound convoluted. Eric -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
