Re: Report Double Fetch Bug Found in Linux-4.6.1/kernel/auditsc.c

On Tue, Jun 21, 2016 at 12:59 PM, Ben Hutchings <ben(a)decadent.org.uk&gt; wrote: > On Tue, 2016-06-21 at 15:18 -0400, Richard Guy Briggs wrote: >> On 2016-06-21 19:20, Ben Hutchings wrote: >> > On Tue, 2016-06-21 at 14:14 -0400, Richard Guy Briggs wrote: >> > > On 2016-06-21 10:51, Ben Hutchings wrote: >> > > > On Tue, 2016-06-21 at 10:37 +0100, Pengfei Wang wrote: >> > > > > > >> > > > > > 在 2016年6月20日，下午8:18，Oleg Nesterov <oleg(a)redhat.com&gt; 写道： >> > > > > > >> > > > > > Not that I understand this report, but >> > > > > > >> > > > > > On 06/20, Richard Guy Briggs wrote: >> > > > > > > >> > > > > > > This function is only ever called by __audit_free(), which is only ever >> > > > > > > called on failure of task creation or on exit of the task, so in neither >> > > > > > > case can anything else change it. >> > > > > > >> > > > > > How so? >> > > > > > >> > > > > > Another thread or CLONE_VM task or /proc/pid/mem can change the user-space >> > > > > > memory in parallel. >> > > > > > >> > > > > > Oleg. >> > > > > >> > > > > >> > > > > Exactly, by saying “change the data”, I mean the modification from >> > > > > malicious users with crafted operations on the user space memory >> > > > > directly, rather than the normal operations within the audit >> > > > > subsystem in Linux. Moreover, since the copy operations from the user >> > > > > space are not protected by any locks or synchronization primitives, >> > > > > changing the data under race condition is feasible I think. Besides, >> > > > > there isn’t any visible checking step in the code to guarantee the >> > > > > consistency between the two copy operations. >> > > > > >> > > > > Here I would like to figure out what the consequences really are once >> > > > > the data is changed between the two copy operations, such as changing >> > > > > a none-control string to a control string but process it as a none- >> > > > > control string that has no control chars. I think problems will >> > > > > happen. >> > > > >> > > > So far as userland can see, kernel log lines are separated by newlines. >> > > >> > > Newlines are control characters that would be caught by that filter. >> > > That filter catches '"', < 0x21, > 0x7e. >> > > >> > > > If we fail to escape a newline, that makes it possible to inject >> > > > arbitrary log lines into the kernel log, which may be misleading to the >> > > > administrator or to software parsing the log. >> > > >> > > So, this is addressed, but I'm still trying to assess the danger of this >> > > repeated call to copy_from_user(). >> > >> > The problem is that newlines can be added to the strings by another >> > task between the first pass that checks for control characters and the >> > second pass that copies them to the log. >> >> Understood, so this is the same sort of problem as Pengfei has raised >> with respect to double quotes being added. >> >> How can subsequent accesses of copy_from_user() be locked, or make sure >> the entire buffer is copied in one go? > > I don't believe it can. And the fact that those strings can be > modified before they're logged kind of defeats the purpose of auditing, > no? Seems like it would make more sense to copy the program name from > the binprm, log that at this point and don't even attempt to log the > arguments. Agreed.

You definintely can't lock the string. An attacker could put the string in MAP_SHARED memory, for example.

--Andy