linux-kernel - Re: [PATCH v2] mm,procfs: allow read-only remote mm access under CAP

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Message-Id: <20250127164106.5f40b62e0f1cf353538c46fd@linux-foundation.org>
Date: Mon, 27 Jan 2025 16:41:06 -0800
From: Andrew Morton <akpm@...ux-foundation.org>
To: Andrii Nakryiko <andrii@...nel.org>
Cc: linux-mm@...ck.org, linux-fsdevel@...r.kernel.org, brauner@...nel.org,
 viro@...iv.linux.org.uk, linux-kernel@...r.kernel.org, bpf@...r.kernel.org,
 kernel-team@...a.com, rostedt@...dmis.org, peterz@...radead.org,
 mingo@...nel.org, linux-trace-kernel@...r.kernel.org,
 linux-perf-users@...r.kernel.org, shakeel.butt@...ux.dev, rppt@...nel.org,
 liam.howlett@...cle.com, surenb@...gle.com, kees@...nel.org,
 jannh@...gle.com
Subject: Re: [PATCH v2] mm,procfs: allow read-only remote mm access under
 CAP_PERFMON

On Mon, 27 Jan 2025 14:21:14 -0800 Andrii Nakryiko <andrii@...nel.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. So /proc/PID/mem, which uses
> PTRACE_MODE_ATTACH, won't be permitted by CAP_PERFMON, but
> /proc/PID/maps, /proc/PID/environ, and a bunch of other read-only
> contents will be allowable under 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.
> 

This should be documented somewhere, so we can tell our users what we
did.  Documentation/filesystems/proc.rst seems to be the place.  .