amd64: do not allow to set reserved bits in MXCSR for ptrace(PT_SETFPREGS)
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Authored by kib on Jul 30 2026, 4:22 AM.

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markj
jhb

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rGcef05c5a62ba: amd64: do not allow to set reserved bits in MXCSR for ptrace(PT_SETFPREGS)

Summary

Also do not mask bits in the mxcsr_mask.  It is ignored by FRSTOR/XRSTOR.

Reported by:    markj

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rG FreeBSD src repository

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Tests Not Applicable

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kib created this revision.Jul 30 2026, 4:22 AM

Herald added a subscriber: imp. · View Herald TranscriptJul 30 2026, 4:22 AM

kib requested review of this revision.Jul 30 2026, 4:22 AM

Have you had a chance to test this under GDB and/or LLDB? I can't imagine they'd be setting any reserved bits so this should be fine.

This revision is now accepted and ready to land.Jul 30 2026, 12:34 PM

markj accepted this revision.Jul 30 2026, 2:52 PM

In D58548#1343721, @jhb wrote:

Have you had a chance to test this under GDB and/or LLDB? I can't imagine they'd be setting any reserved bits so this should be fine.

In such case we would get the #GP I believe, for the same reason as in the Mark' syzcaller reproducer.

Closed by commit rGcef05c5a62ba: amd64: do not allow to set reserved bits in MXCSR for ptrace(PT_SETFPREGS) (authored by kib). · Explain WhyJul 30 2026, 5:09 PM

This revision was automatically updated to reflect the committed changes.

kib added a commit: rGcef05c5a62ba: amd64: do not allow to set reserved bits in MXCSR for ptrace(PT_SETFPREGS).

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sys/

amd64/

exec_machdep.c

17 lines

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sys/amd64/amd64/exec_machdep.c

amd64: do not allow to set reserved bits in MXCSR for ptrace(PT_SETFPREGS)ClosedPublicActions