Messages in this thread |  | | Date | Sat, 21 Jan 2023 19:28:15 -0800 | From | "H. Peter Anvin" <> | Subject | RE: the x86 sysret_rip test fails on the Intel FRED architecture |
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On January 21, 2023 7:01:53 PM PST, "Li, Xin3" <xin3.li@intel.com> wrote: >> > >> If not intentional, it might be something that can still be fixed. >> > >> If it is intentional and is going to be with us for a while we have >> > >> a few options. If userspace is _really_ depending on this >> > >> behavior, we could just clobber r11 ourselves in the FRED entry >> > >> path. If not, we can remove the assertion in the selftest. >> > > We can't clobber it in the FRED entry path, since it is common for >> > > all events, but we could do it in the syscall dispatch. >> > > >> > > However, it doesn't seem to make sense to do so to me. The current >> > > behavior is much more of an artifact than desired behavior. >> > I guess the SDM statements really are for the kernel's benefit and not >> > for userspace. Userspace _should_ be treating SYSCALL like a CALL and >> > r11 like any old register that can be clobbered. Right now, the >> > kernel just happens to clobber it with RFLAGS. >> > >> > I do the the odds of anyone relying on this behavior are pretty small. >> > Let's just zap the check from the selftest, document what we did in >> > the FRED docs and changelog and move on. >> >> Keep the selftest check, but also accept preserved RCX/R11. What really matters is >> that the kernel isn't leaking data. > >I feel it the same way, it looks to me that the check is to make sure >R11 doesn't leak any kernel data because the Linux kernel deliberately >overwrites R11 with the value of user level flags just before returning >to user level. > >I wanted to zap the check, but as HPA said, this is an artifact to not leak >any kernel data. I guess it doesn't make a difference if the kernel sets >R11 to 0. > >Maybe it's still reasonable to keep such a check for IDT. However, it makes >no sense for FRED systems, because all GP registers are saved/restored upon >event delivery/return. > >Thanks! > Xin > >> >> -- >> Brian Gerst >
The big thing is that the system calls that return with sysret v iret on IDT systems need to be consistent, in order to not leak kernel state.
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