Messages in this thread | | | Date | Mon, 9 Jan 2023 15:31:56 +0000 | From | Mark Rutland <> | Subject | Re: Expected rdpmc behavior during context swtich and a RISC-V conundrum |
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On Mon, Jan 09, 2023 at 01:41:15PM +0100, Peter Zijlstra wrote: > On Thu, Jan 05, 2023 at 11:59:24AM -0800, Atish Patra wrote: > > Hi All, > > There was a recent uabi update[1] for RISC-V that allows the users to > > read cycle and instruction count without any checks. > > We tried to restrict that behavior to address security concerns > > earlier but it resulted in breakage for some user space > > applications[2]. > > Thus, previous behavior was restored where a user on RISC-V platforms > > can directly read cycle or instruction count[3]. > > > > Comparison with other ISAs w.r.t user space access of counters: > > ARM64 > > -- Enabled/Disabled via (/proc/sys/kernel/perf_user_access) > > -- Only for task bound events configured via perf. > > > > X86 > > --- rdpmc instruction > > --- Enable/Disable via “/sys/devices/cpu/rdpmc” > > -- Before v4.0 > > -- any process (even without active perf event) rdpmc > > After v4.0 > > -- Default behavior changed to support only active events in a > > process’s context. > > -- Configured through perf similar to ARM64 > > -- Continue to maintain backward compatibility for unrestricted access > > by writing 2 to “/sys/devices/cpu/rdpmc” > > > > IMO, RISC-V should only enable user space access through perf similar > > to ARM64 and x86 (post v4.0). > > However, we do have to support the legacy behavior to avoid > > application breakage. > > As per my understanding a direct user space access can lead to the > > following problems: > > > > 1) There is no context switch support, so counts from other contexts are exposed > > 2) If a perf user is allocated one of these counters, the counter > > value will be written > > > > Looking at the x86 code as it continues to allow the above behavior, > > rdpmc_always_available_key is enabled in the above case. However, > > during the context switch (cr4_update_pce_mm) > > only dirty counters are cleared. It only prevents leakage from perf > > task to rdpmc task. > > > > How does the context switch of counters work for users who enable > > unrestricted access by writing 2 to “/sys/devices/cpu/rdpmc” ? > > Otherwise, rdpmc users likely get noise from other applications. Is > > that expected ? > > This can be a security concern also where a rogue rdpmc user > > application can monitor other critical applications to initiate side > > channel attack. > > > > Am I missing something? Please correct my understanding of the x86 > > implementation if it is wrong. > > So on x86 we have RDTSC and RDPMC instructions. RDTSC reads the > Time-Stamp-Counter which is a globally synchronized monotonic increasing > counter at some 'random' rate (idealized, don't ask). This thing is used > for time-keeping etc..
For context, the arm64 equivalent would be CNTVCT_EL0, which is a constant-rate always-on free-running counter which is (architecturally) consistent across CPUs, whereas PMCCNTR_EL0 is not any of those things.
> And then there's RDPMC which (optionally) allows reading the PMU > counters which are normally disabled and all 0. > > Even if RDPMC is unconditionally allowed from userspace (the 2 option > you refer to) userspace will only be able to read these 0s unless > someone also programs the PMU. Linux only supports a single means of > doing so: perf (some people use /dev/msr to poke directly to the MSRs > but they get to keep all pieces). > > RDPMC is only useful if you read counters you own on yourself -- IOW > selfmonitoring, using the interface outlined in uapi/linux/perf_events.h > near struct perf_event_mmap_page. > > Any other usage -- you get to keep the pieces.
Yup.
> Can you observe random other counters, yes, unavoidably so. The sysfs > control you mention was instituted to restrict this somewhat. > > If the RISC-V counters are fundamentally the PMU counters that need to > be reset to trigger events, then you've managed to paint yourself into a > tight spot :/ > > Either you must dis-allow userspace access to these things (and break > them) or limit the PMU usage -- both options suck.
> Now, I'm thinking that esp. something like instruction count is not > synchronized between cores (seems fundamentally impossible) and can only > be reasonably be consumed (and compared) when strictly affine to a > particular CPU, you can argue that applications doing this without also > strictly managing their affinity mask are broken anyway and therefore > your breakage is not in fact a breaking them -- you can't break > something that's already broken.
Yup, that was my thinking too.
The intermediate option is to trap-and-emulate (as zero or some other fixed value), which highlghts the bug without crashing applications.
> > Anyway, given RISC-V being a very young platform, I would try really > *really* *REALLY* hard to stomp on these applications and get them to > change in order to reclaim the PMU usage.
Agreed.
Thanks, Mark.
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