Messages in this thread Patch in this message | | | Date | Tue, 25 Jan 2022 11:12:45 +0100 | From | Borislav Petkov <> | Subject | Re: [PATCH] x86/fpu: remove memset call for xmm registers on xfpregs_set |
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On Tue, Jan 25, 2022 at 02:20:15AM +0000, Luís Ferreira wrote: > This patch fixes https://bugzilla.kernel.org/show_bug.cgi?id=215524 by removing > the memset introduced by 6164331d15f7d912fb9369245368e9564ea49813, which > created a regression on ptrace PTRACE_SETREGSET request with NT_FPREGSET.
Is NT_FPREGSET the same as NT_PRFPREG core_note_type?
I can see former is mentioned in elf(5) as an ELF note type:
/usr/include/elf.h: #define NT_FPREGSET 2 /* Contains copy of fpregset struct */
while latter is defined in the kernel tree like this:
/* * Notes used in ET_CORE. Architectures export some of the arch register sets * using the corresponding note types via the PTRACE_GETREGSET and * PTRACE_SETREGSET requests. * The note name for all these is "LINUX". */ #define NT_PRSTATUS 1 #define NT_PRFPREG 2 ...
I guess those are one and the same thing - just named differently.
> Particularly, it zeros some XMM registers on the wrong offsets. Fixing the offsets > only solves the problem for i686, which doesn't include xmm8-15 registers, so > the right way is to probably completely remove this call.
It's a good thing you mention i686. I believe the reason for the zeroing was the X86_FEATURE_FXSR check which means the CPU has FXSAVE/FXRSTOR, CR4.OSFXSR support.
Now, FXSAVE doc says:
"The architecture supports two 512-bit memory formats for FXSAVE, a 64-bit format that saves XMM0-XMM15, and a 32-bit legacy format that saves only XMM0-XMM7. If FXSAVE is executed in 64-bit mode, the 64-bit format is used, otherwise the 32-bit format is used."
Then, just to check I went and built a kernel before:
6164331d15f7 ("x86/fpu: Rewrite xfpregs_set()")
and tried the reproducer with %xmm7 and %xmm9:
$ gdb /bin/bash -batch -x gdbcmds warning: Could not load shared library symbols for linux-vdso.so.1. Do you need "set solib-search-path" or "set sysroot"?
Program received signal SIGILL, Illegal instruction. 0x00007ffff79d8a97 in kill () from /lib/x86_64-linux-gnu/libc.so.6 $1 = 0x20202020202020202020202020202020 Setting xmm7 to 35322350018591 $2 = 35322350018591
$ gdb /bin/bash -batch -x gdbcmds warning: Could not load shared library symbols for linux-vdso.so.1. Do you need "set solib-search-path" or "set sysroot"?
Program received signal SIGILL, Illegal instruction. 0x00007ffff79d8a97 in kill () from /lib/x86_64-linux-gnu/libc.so.6 $1 = 0xffffffffffffffffffffffffffffffff Setting xmm9 to 35322350018591 $2 = 35322350018591
So whatever that clearing was supposed to do, it needs to go because it used to work before that so this looks like a regression. Unless I'm missing an aspect...
> Signed-off-by: Luís Ferreira <contact@lsferreira.net> > --- > arch/x86/kernel/fpu/regset.c | 2 -- > 1 file changed, 2 deletions(-) > > diff --git a/arch/x86/kernel/fpu/regset.c b/arch/x86/kernel/fpu/regset.c > index 437d7c930c0b..aec6c49029b1 100644 > --- a/arch/x86/kernel/fpu/regset.c > +++ b/arch/x86/kernel/fpu/regset.c > @@ -116,9 +116,7 @@ int xfpregs_set(struct task_struct *target, const struct user_regset *regset, > /* Copy the state */ > memcpy(&fpu->fpstate->regs.fxsave, &newstate, sizeof(newstate)); > > - /* Clear xmm8..15 */ > BUILD_BUG_ON(sizeof(fpu->__fpstate.regs.fxsave.xmm_space) != 16 * 16); > - memset(&fpu->fpstate->regs.fxsave.xmm_space[8], 0, 8 * 16); > > /* Mark FP and SSE as in use when XSAVE is enabled */ > if (use_xsave()) > --
-- Regards/Gruss, Boris.
https://people.kernel.org/tglx/notes-about-netiquette
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