Re: [PATCH] mm,procfs: allow read-only remote mm access under CAP_PERFMON

[Jann Horn <jannh@google.com>]

On Fri, Jan 24, 2025 at 12:47 AM Kees Cook <kees@kernel.org> wrote:
> On Thu, Jan 23, 2025 at 01:52:52PM -0800, Suren Baghdasaryan wrote:
> > On Thu, Jan 23, 2025 at 1:44 PM Andrii Nakryiko <andrii@kernel.org> wrote:
> > >
> > > It's very common for various tracing and profiling toolis to need to
> > > access /proc/PID/maps contents for stack symbolization needs to learn
> > > which shared libraries are mapped in memory, at which file offset, etc.
> > > Currently, access to /proc/PID/maps requires CAP_SYS_PTRACE (unless we
> > > are looking at data for our own process, which is a trivial case not too
> > > relevant for profilers use cases).
> > >
> > > Unfortunately, CAP_SYS_PTRACE implies way more than just ability to
> > > discover memory layout of another process: it allows to fully control
> > > arbitrary other processes. This is problematic from security POV for
> > > applications that only need read-only /proc/PID/maps (and other similar
> > > read-only data) access, and in large production settings CAP_SYS_PTRACE
> > > is frowned upon even for the system-wide profilers.
> > >
> > > On the other hand, it's already possible to access similar kind of
> > > information (and more) with just CAP_PERFMON capability. E.g., setting
> > > up PERF_RECORD_MMAP collection through perf_event_open() would give one
> > > similar information to what /proc/PID/maps provides.
> > >
> > > CAP_PERFMON, together with CAP_BPF, is already a very common combination
> > > for system-wide profiling and observability application. As such, it's
> > > reasonable and convenient to be able to access /proc/PID/maps with
> > > CAP_PERFMON capabilities instead of CAP_SYS_PTRACE.
> > >
> > > For procfs, these permissions are checked through common mm_access()
> > > helper, and so we augment that with cap_perfmon() check *only* if
> > > requested mode is PTRACE_MODE_READ. I.e., PTRACE_MODE_ATTACH wouldn't be
> > > permitted by CAP_PERFMON.
> > >
> > > Besides procfs itself, mm_access() is used by process_madvise() and
> > > process_vm_{readv,writev}() syscalls. The former one uses
> > > PTRACE_MODE_READ to avoid leaking ASLR metadata, and as such CAP_PERFMON
> > > seems like a meaningful allowable capability as well.
> > >
> > > process_vm_{readv,writev} currently assume PTRACE_MODE_ATTACH level of
> > > permissions (though for readv PTRACE_MODE_READ seems more reasonable,
> > > but that's outside the scope of this change), and as such won't be
> > > affected by this patch.
> >
> > CC'ing Jann and Kees.
> >
> > >
> > > Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
> > > ---
> > >  kernel/fork.c | 11 ++++++++++-
> > >  1 file changed, 10 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/kernel/fork.c b/kernel/fork.c
> > > index ded49f18cd95..c57cb3ad9931 100644
> > > --- a/kernel/fork.c
> > > +++ b/kernel/fork.c
> > > @@ -1547,6 +1547,15 @@ struct mm_struct *get_task_mm(struct task_struct *task)
> > >  }
> > >  EXPORT_SYMBOL_GPL(get_task_mm);
> > >
> > > +static bool can_access_mm(struct mm_struct *mm, struct task_struct *task, unsigned int mode)
> > > +{
> > > +       if (mm == current->mm)
> > > +               return true;
> > > +       if ((mode & PTRACE_MODE_READ) && perfmon_capable())
> > > +               return true;
> > > +       return ptrace_may_access(task, mode);
> > > +}
>
> nit: "may" tends to be used more than "can" for access check function naming.
>
> So, this will bypass security_ptrace_access_check() within
> ptrace_may_access(). CAP_PERFMON may be something LSMs want visibility
> into.
>
> It also bypasses the dumpability check in __ptrace_may_access(). (Should
> non-dumpability block visibility into "maps" under CAP_PERFMON?)
>
> This change provides read access for CAP_PERFMON to:
>
> /proc/$pid/maps
> /proc/$pid/smaps
> /proc/$pid/mem
> /proc/$pid/environ
> /proc/$pid/auxv
> /proc/$pid/attr/*
> /proc/$pid/smaps_rollup
> /proc/$pid/pagemap
>
> /proc/$pid/mem access seems way out of bounds for CAP_PERFMON. environ
> and auxv maybe too much also. The "attr" files seem iffy. pagemap may be
> reasonable.

FWIW, my understanding is that if you can use perf_event_open() on a
process, you can also grab large amounts of stack memory contents from
that process via PERF_SAMPLE_STACK_USER/sample_stack_user. (The idea
there is that stack unwinding for userspace stacks is complicated, so
it's the profiler's job to turn a pile of raw stack contents and a
register snapshot into a stack trace.) So _to some extent_ I think it
is already possible to read memory of another process via CAP_PERFMON.
Whether that is desirable or not I don't know, though I guess it's
hard to argue that there's a qualitative security difference between
reading register contents and reading stack memory...