Messages in this thread | | | Subject | Re: [PATCH v9 15/29] x86/insn-eval: Add utility functions to get segment descriptor base address and limit | From | Ricardo Neri <> | Date | Wed, 11 Oct 2017 18:24:03 -0700 |
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On Wed, 2017-10-11 at 22:16 +0200, Borislav Petkov wrote: > On Wed, Oct 11, 2017 at 12:57:01PM -0700, Ricardo Neri wrote: > > > > This is meant to be an error case. In long mode, > > only INAT_SEG_REG_IGNORE/FS/GS > > are valid. All other indices are invalid. > > > > Perhaps we could return -EINVAL instead? > So, my question is, when are you ever going to have that case? What > constellation of events would ever hit this else branch for long mode? > Because it looks impossible to me. What I can imagine only is something > like this: > > else if (seg_reg != INAT_SEG_REG_IGNORE) > WARN_ONCE(1, "This should never happen!\n"); > > assertion.
To clarify, I think you mean seg_reg_idx.
Yes, it would be impossible to hit this else branch provided that callers don't attempt to use an invalid seg_reg_idx while in long mode. Probably this is not critical as this is a static function and as such we control who can call it and make sure seg_reg_idx is always valid (i.e., INAT_SEG_REG_IGNORE/FS/GS in long mode).
> But you don't really need that - you can simply ignore seg_reg in that > case: > > if (user_64bit_mode(regs)) { > /* > * Only FS or GS will have a base address, the rest of > * the segments' bases are forced to 0. > */ > unsigned long base; > > if (seg_reg == INAT_SEG_REG_FS) > rdmsrl(MSR_FS_BASE, base); > else if (seg_reg == INAT_SEG_REG_GS) > /* > * swapgs was called at the kernel entry point. Thus, > * MSR_KERNEL_GS_BASE will have the user-space GS > base. > */ > rdmsrl(MSR_KERNEL_GS_BASE, base); > else > base = 0; > > return base; > } > > Or am I missing something?
My intention is to let the caller know about the invalid seg_reg_idx instead of silently correcting the caller's input by ignoring seg_reg_idx.
On the other hand, in long mode, hardware ignore all segment registers except FS and GS.
Hence, I guess I can remove the check in question.
Thanks and BR, Ricardo
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