On Thu, Jan 23, 2025 at 3:47 PM Kees Cook [off-list ref] wrote:

On Thu, Jan 23, 2025 at 01:52:52PM -0800, Suren Baghdasaryan wrote:

quoted

On Thu, Jan 23, 2025 at 1:44 PM Andrii Nakryiko [off-list ref] wrote:

quoted

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.

quoted

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.

good point, will change to "may"

So, this will bypass security_ptrace_access_check() within
ptrace_may_access(). CAP_PERFMON may be something LSMs want visibility
into.

yeah, similar to perf's perf_check_permission() (though, admittedly,
perf has its own security_perf_event_open(&attr, PERF_SECURITY_OPEN)
check much earlier in perf_event_open() logic)

It also bypasses the dumpability check in __ptrace_may_access(). (Should
non-dumpability block visibility into "maps" under CAP_PERFMON?)

With perf_event_open() and PERF_RECORD_MMAP none of this dumpability
is honored today as well, so I think CAP_PERFMON should override all
these ptrace things here, no?

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.

As Shakeel pointed out, /proc/PID/mem is PTRACE_MODE_ATTACH, so won't
be permitted under CAP_PERFMON either.

Don't really know what auxv is, but I could read all that with BPF if
I had CAP_PERFMON, for any task, so not like we are opening up new
possibilities here.

Gaining CAP_PERFMON access to *only* the "maps" file doesn't seem too
bad to me, but I think the proposed patch ends up providing way too wide
access to other things.

I do care about maps mostly, yes, but I also wanted to avoid
duplicating all that mm_access() logic just for maps (and probably
smaps, they are the same data). But again, CAP_PERFMON basically means
read-only tracing access to anything within kernel and any user
process, so it felt appropriate to allow CAP_PERFMON here.

Also, this is doing an init-namespace capability check for
CAP_PERFMON (via perfmon_capable()). Shouldn't this be per-namespace?

CAP_PERFMON isn't namespaced as far as perf_event_open() is concerned,
so at least for that reason I don't want to relax the requirement
here. Namespacing CAP_PERFMON in general is interesting and I bet
there are users that would appreciate that, but that's an entire epic
journey we probably don't want to start here.

-Kees

quoted

quoted

+
 struct mm_struct *mm_access(struct task_struct *task, unsigned int mode)
 {
        struct mm_struct *mm;

@@ -1559,7 +1568,7 @@ struct mm_struct *mm_access(struct task_struct *task, unsigned int mode)
        mm = get_task_mm(task);
        if (!mm) {
                mm = ERR_PTR(-ESRCH);
-       } else if (mm != current->mm && !ptrace_may_access(task, mode)) {
+       } else if (!can_access_mm(mm, task, mode)) {
                mmput(mm);
                mm = ERR_PTR(-EACCES);
        }
--

2.43.5

--
Kees Cook