From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id BCF7F1BDAB5; Thu, 23 Jan 2025 23:47:47 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1737676067; cv=none; b=oS9295GuKgG0m8dYqHhTyTfn1oeBqsz3bOAiC+UaFKXENTAVVQeQaLPK6zWt1ItCvvEhm21CgbhYaiP2ipstd55B7AXUPfMpLo6WdOUsyk6U5VHfA2J/pr7alg+vinv+2r3eXhzY2mrYqDHOiW8n9vrAQxOpc0+0z1y2CQVeT9U= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1737676067; c=relaxed/simple; bh=wraYd2NGwuEyzk8Cgc5yaLobelCCviWThCaaKkdHMu8=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=SH4/Jxmvow2rSM4DXVkMPChIlVUEbuAfJ+fXhvJLsLi7Yv+LZgdfFb9XQbmTwG+mtAgVv7krducSGbRx1BISaXSZHHQodn4j8B/wvPFNkvOPnhkpV+j1ZqCEaX1Zy4puOCciFEbQQbum/hmas2toSc3UMoyDMuKdmWSvY7sYSKI= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=uoM1KAGZ; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="uoM1KAGZ" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 29BCFC4CED3; Thu, 23 Jan 2025 23:47:47 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1737676067; bh=wraYd2NGwuEyzk8Cgc5yaLobelCCviWThCaaKkdHMu8=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=uoM1KAGZIiefDfHgZDxyELPBM22hTQH1XyHU+IpjJAIv2QNNwCPYMVqdM+htvYa6w d/Un2AKdgpKpInZhod29JdKJpafmOZ8d2d57cmMi1Akm2/cDrq5gmDCA3LgeTI3DUU dDY7Lz42F4b4Md8af6BpBtGZQ+loK2dOPaBW/haDChHwnz2lrvR6Fels6V9xvTvTsw CtGcpS4SVKObhi7BWGvmLIhQAXGXwAykmaj7bc4onqYX6qyotevurEzmT0FDC80Fgg xlGnalQak8rVIzU7ViSZumFDib6h7XwqcElLIpitqYAGiiIRWbQzzhPA03Vg5ERZhC dxgg1WiZJ0z0A== Date: Thu, 23 Jan 2025 15:47:44 -0800 From: Kees Cook To: Suren Baghdasaryan Cc: Andrii Nakryiko , linux-mm@kvack.org, akpm@linux-foundation.org, linux-fsdevel@vger.kernel.org, brauner@kernel.org, viro@zeniv.linux.org.uk, linux-kernel@vger.kernel.org, bpf@vger.kernel.org, kernel-team@meta.com, rostedt@goodmis.org, peterz@infradead.org, mingo@kernel.org, linux-trace-kernel@vger.kernel.org, linux-perf-users@vger.kernel.org, shakeel.butt@linux.dev, rppt@kernel.org, liam.howlett@oracle.com, Jann Horn , linux-security-module@vger.kernel.org Subject: Re: [PATCH] mm,procfs: allow read-only remote mm access under CAP_PERFMON Message-ID: <202501231526.A3C13EC5@keescook> References: <20250123214342.4145818-1-andrii@kernel.org> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: On Thu, Jan 23, 2025 at 01:52:52PM -0800, Suren Baghdasaryan wrote: > On Thu, Jan 23, 2025 at 1:44 PM Andrii Nakryiko 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 > > --- > > 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. 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. Also, this is doing an init-namespace capability check for CAP_PERFMON (via perfmon_capable()). Shouldn't this be per-namespace? -Kees > > + > > 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