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 12:57:01 -0700 |
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On Wed, 2017-10-11 at 17:15 +0200, Borislav Petkov wrote: > On Tue, Oct 03, 2017 at 08:54:18PM -0700, Ricardo Neri wrote: > > > > With segmentation, the base address of the segment is needed to compute a > > linear address. This base address is obtained from the applicable segment > > descriptor. Such segment descriptor is referenced from a segment selector. > ... > > > > > +unsigned long insn_get_seg_base(struct pt_regs *regs, int seg_reg_idx) > > +{ > > + struct desc_struct *desc; > > + short sel; > > + > > + sel = get_segment_selector(regs, seg_reg_idx); > > + if (sel < 0) > > + return -1L; > > + > > + if (v8086_mode(regs)) > > + /* > > + * Base is simply the segment selector shifted 4 > > + * positions to the right. > > + */ > > + return (unsigned long)(sel << 4); > > + > > + 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_idx == INAT_SEG_REG_FS) > > + rdmsrl(MSR_FS_BASE, base); > > + else if (seg_reg_idx == 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 if (seg_reg_idx != INAT_SEG_REG_IGNORE) > > + /* We should ignore the rest of segment registers. > > */ > > + base = -1L; > When is that case ever possible in long mode? You either have GS/FS > bases or 0. What's the meaning of -1L in that case?
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?
> Otherwise just minor things: > > --- > diff --git a/arch/x86/lib/insn-eval.c b/arch/x86/lib/insn-eval.c > index 7ba5379a2923..e7e82b343bd0 100644 > --- a/arch/x86/lib/insn-eval.c > +++ b/arch/x86/lib/insn-eval.c > @@ -515,13 +515,13 @@ static struct desc_struct *get_desc(unsigned short sel) > * > * Obtain the base address of the segment as indicated by the segment > descriptor > * pointed by the segment selector. The segment selector is obtained from the > - * input segment register index seg_reg_idx. > + * input segment register index @seg_reg_idx. > * > * Returns: > * > * In protected mode, base address of the segment. Zero in long mode, > * except when FS or GS are used. In virtual-8086 mode, the segment > - * selector shifted 4 positions to the right. > + * selector shifted 4 bits to the right. > * > * -1L in case of error. > */ > @@ -537,7 +537,7 @@ unsigned long insn_get_seg_base(struct pt_regs *regs, int > seg_reg_idx) > if (v8086_mode(regs)) > /* > * Base is simply the segment selector shifted 4 > - * positions to the right. > + * bits to the right. > */ > return (unsigned long)(sel << 4);
Thanks! I will incorporate these changes along with your Improvements-by tag.
BR, Ricardo
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