Messages in this thread Patch in this message | | | Date | Wed, 11 Oct 2017 17:15:18 +0200 | From | Borislav Petkov <> | Subject | Re: [PATCH v9 15/29] x86/insn-eval: Add utility functions to get segment descriptor base address and limit |
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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?
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); -- Regards/Gruss, Boris. SUSE Linux GmbH, GF: Felix Imendörffer, Jane Smithard, Graham Norton, HRB 21284 (AG Nürnberg) --
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