On Wed 20-01-21 14:17:39, Jann Horn wrote: > On Wed, Jan 13, 2021 at 3:22 PM Michal Hocko <mhocko@xxxxxxxx> wrote: > > On Tue 12-01-21 09:51:24, Suren Baghdasaryan wrote: > > > On Tue, Jan 12, 2021 at 9:45 AM Oleg Nesterov <oleg@xxxxxxxxxx> 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. > > Yama in particular only does its checks on ATTACH and ignores READ, > that's the difference you're probably most likely to encounter on a > normal desktop system, since some distros turn Yama on by default. > Basically the idea there is that running "gdb -p $pid" or "strace -p > $pid" as a normal user will usually fail, but reading /proc/$pid/maps > still works; so you can see things like detailed memory usage > information and such, but you're not supposed to be able to directly > peek into a running SSH client and inject data into the existing SSH > connection, or steal the cryptographic keys for the current > connection, or something like that. > > > > 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. > > > > Maybe this would be a good start to document expectations. Some more > > practical examples where the difference is visible would be great as > > well. > > Before documenting the behavior, it would be a good idea to figure out > what to do with perf_event_open(). That one's weird in that it only > requires PTRACE_MODE_READ, but actually allows you to sample stuff > like userspace stack and register contents (if perf_event_paranoid is > 1 or 2). Maybe for SELinux things (and maybe also for Yama), there > should be a level in between that allows fully inspecting the process > (for purposes like profiling) but without the ability to corrupt its > memory or registers or things like that. Or maybe perf_event_open() > should just use the ATTACH mode. Thanks for the clarification. I still cannot say I would have a good mental picture. Having something in Documentation/core-api/ sounds really needed. Wrt to perf_event_open it sounds really odd it can do more than other places restrict indeed. Something for the respective maintainer but I strongly suspect people simply copy the pattern from other places because the expected semantic is not really clear. -- Michal Hocko SUSE Labs
