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Date: Tue, 12 Jan 2021 09:51:24 -0800 From: Suren Baghdasaryan <surenb@...gle.com> To: Oleg Nesterov <oleg@...hat.com> Cc: Michal Hocko <mhocko@...e.com>, Andrew Morton <akpm@...ux-foundation.org>, Jann Horn <jannh@...gle.com>, Kees Cook <keescook@...omium.org>, Jeffrey Vander Stoep <jeffv@...gle.com>, Minchan Kim <minchan@...nel.org>, Shakeel Butt <shakeelb@...gle.com>, David Rientjes <rientjes@...gle.com>, Edgar Arriaga GarcĂa <edgararriaga@...gle.com>, Tim Murray <timmurray@...gle.com>, linux-mm <linux-mm@...ck.org>, selinux@...r.kernel.org, Linux API <linux-api@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>, kernel-team <kernel-team@...roid.com> Subject: Re: [PATCH v2 1/1] mm/madvise: replace ptrace attach requirement for process_madvise On Tue, Jan 12, 2021 at 9:45 AM Oleg Nesterov <oleg@...hat.com> wrote: > > On 01/12, Michal Hocko wrote: > > > > On Mon 11-01-21 09:06:22, Suren Baghdasaryan wrote: > > > > > What we want is the ability for one process to influence another process > > > in order to optimize performance across the entire system while leaving > > > the security boundary intact. > > > Replace PTRACE_MODE_ATTACH with a combination of PTRACE_MODE_READ > > > and CAP_SYS_NICE. PTRACE_MODE_READ to prevent leaking ASLR metadata > > > and CAP_SYS_NICE for influencing process performance. > > > > I have to say that ptrace modes are rather obscure to me. So I cannot > > really judge whether MODE_READ is sufficient. My understanding has > > always been that this is requred to RO access to the address space. But > > this operation clearly has a visible side effect. Do we have any actual > > documentation for the existing modes? > > > > I would be really curious to hear from Jann and Oleg (now Cced). > > Can't comment, sorry. I never understood these security checks and never tried. > IIUC only selinux/etc can treat ATTACH/READ differently and I have no idea what > is the difference. I haven't seen a written explanation on ptrace modes but when I consulted Jann his explanation was: PTRACE_MODE_READ means you can inspect metadata about processes with the specified domain, across UID boundaries. PTRACE_MODE_ATTACH means you can fully impersonate processes with the specified domain, across UID boundaries. He did agree that in this case PTRACE_MODE_ATTACH seems too restrictive (we do not try to gain full control or impersonate a process) and PTRACE_MODE_READ is a better choice. > > Oleg. >
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