Messages in this thread | | | Date | Thu, 17 Sep 2020 19:28:40 +0200 | From | Borislav Petkov <> | Subject | Re: [PATCH] x86/cmdline: Disable instrumentation of cmdline unconditionally |
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On Thu, Sep 17, 2020 at 12:05:48PM -0400, Arvind Sankar wrote: > I did have a crash and this patch fixed it, but it seems it was on a > branch where I was making changes to cmdline.c, which triggered clang to > use a jump table for cmdline_find_option_bool(). That was the cause of > the crash, and the reason this patch fixed it was because it enabled > -fno-jump-tables, rather than because it disabled instrumentation.
I see.
> The instrumentation code does write data to random addresses, but > apparently that doesn't necessarily crash the system. This patch would > also be insufficient to fix it, since load_ucode_bsp() itself can have > instrumentation code in it.
Yeah, it better not have any.
> Eg with GCOV_PROFILE_ALL enabled, the start of the function is: > > c2a7706a <load_ucode_bsp>: > c2a7706a: 55 push %ebp > c2a7706b: 83 05 c0 4d ba c2 01 addl $0x1,0xc2ba4dc0 > c2a77072: 83 15 c4 4d ba c2 00 adcl $0x0,0xc2ba4dc4 > c2a77079: 89 e5 mov %esp,%ebp > > but when it's called from arch/x86/kernel/head_32.S, paging is disabled > and the code is executing out of physical addresses, so it's going to > read/write data from garbage addresses.
Right, so I'm sceptical to all this instrumentation nonsense - it is fine and good if you have it but not everywhere. And certainly not when the thing is loading microcode.
microcode/core.c contains all the code, early and late so it might make some little sense to have instrumentation there for whatever reasons, say KASANing (yah, I made a verb :)) the thing for leaks or whatnot, but meh, I can't find it in me to care. So at some point, if it starts getting in the way, we might remove all instrumentation from that thing altogether.
> Anyway, please ignore this patch and sorry for the noise.
No worries.
Thx.
-- Regards/Gruss, Boris.
https://people.kernel.org/tglx/notes-about-netiquette
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