Re: [v6 PATCH 06/21] x86/insn-eval: Add utility functions to get segment selector

Borislav Petkov
2017-04-18T09:42:21Z

On Tue, Mar 07, 2017 at 04:32:39PM -0800, Ricardo Neri wrote:
> When computing a linear address and segmentation is used, we need to know
> the base address of the segment involved in the computation. In most of
> the cases, the segment base address will be zero as in USER_DS/USER32_DS.
> However, it may be possible that a user space program defines its own
> segments via a local descriptor table. In such a case, the segment base
> address may not be zero .Thus, the segment base address is needed to
> calculate correctly the linear address.
>
> The segment selector to be used when computing a linear address is
> determined by either any of segment select override prefixes in the
> instruction or inferred from the registers involved in the computation of
> the effective address; in that order. Also, there are cases when the
> overrides shall be ignored.
>
> For clarity, this process can be split into two steps: resolving the
> relevant segment and, once known, read the applicable segment selector.
> The method to obtain the segment selector depends on several factors. In
> 32-bit builds, segment selectors are saved into the pt_regs structure
> when switching to kernel mode. The same is also true for virtual-8086
> mode. In 64-bit builds, segmentation is mostly ignored, except when
> running a program in 32-bit legacy mode. In this case, CS and SS can be
> obtained from pt_regs. DS, ES, FS and GS can be read directly from
> registers.

> Lastly, segmentation is possible in 64-bit mode via FS and GS.

I'd say "Lastly, the only two segment registers which are not ignored in
long mode are FS and GS."

> In these two cases, base addresses are obtained from the relevant MSRs.

s/relevant/respective/

> Cc: Dave Hansen <dave.hansen@linux.intel.com>
> Cc: Adam Buchbinder <adam.buchbinder@gmail.com>
> Cc: Colin Ian King <colin.king@canonical.com>
> Cc: Lorenzo Stoakes <lstoakes@gmail.com>
> Cc: Qiaowei Ren <qiaowei.ren@intel.com>
> Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
> Cc: Masami Hiramatsu <mhiramat@kernel.org>
> Cc: Adrian Hunter <adrian.hunter@intel.com>
> Cc: Kees Cook <keescook@chromium.org>
> Cc: Thomas Garnier <thgarnie@google.com>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Borislav Petkov <bp@suse.de>
> Cc: Dmitry Vyukov <dvyukov@google.com>
> Cc: Ravi V. Shankar <ravi.v.shankar@intel.com>
> Cc: x86@kernel.org
> Signed-off-by: Ricardo Neri <ricardo.neri-calderon@linux.intel.com>
> ---
> arch/x86/lib/insn-eval.c | 195 +++++++++++++++++++++++++++++++++++++++++++++++
> 1 file changed, 195 insertions(+)
>
> diff --git a/arch/x86/lib/insn-eval.c b/arch/x86/lib/insn-eval.c
> index 78df1c9..8d45df8 100644
> --- a/arch/x86/lib/insn-eval.c
> +++ b/arch/x86/lib/insn-eval.c
> @@ -8,6 +8,7 @@
> #include <asm/inat.h>
> #include <asm/insn.h>
> #include <asm/insn-eval.h>
> +#include <asm/vm86.h>
>
> enum reg_type {
> REG_TYPE_RM = 0,
> @@ -15,6 +16,200 @@ enum reg_type {
> REG_TYPE_BASE,
> };
>
> +enum segment {
> + SEG_CS = 0x23,
> + SEG_SS = 0x36,
> + SEG_DS = 0x3e,
> + SEG_ES = 0x26,
> + SEG_FS = 0x64,
> + SEG_GS = 0x65
> +};
> +
> +/**
> + * resolve_seg_selector() - obtain segment selector
> + * @regs: Set of registers containing the segment selector

That arg is gone.

> + * @insn: Instruction structure with selector override prefixes
> + * @regoff: Operand offset, in pt_regs, of which the selector is needed
> + * @default: Resolve default segment selector (i.e., ignore overrides)
> + *
> + * The segment selector to which an effective address refers depends on
> + * a) segment selector overrides instruction prefixes or b) the operand
> + * register indicated in the ModRM or SiB byte.
> + *
> + * For case a), the function inspects any prefixes in the insn instruction;

s/insn //

> + * insn can be null to indicate that selector override prefixes shall be
> + * ignored.

This is not what the code does: it returns -EINVAL when insn is NULL.

> This is useful when the use of prefixes is forbidden (e.g.,
> + * obtaining the code selector). For case b), the operand register shall be
> + * represented as the offset from the base address of pt_regs. Also, regoff
> + * can be -EINVAL for cases in which registers are not used as operands (e.g.,
> + * when the mod and r/m parts of the ModRM byte are 0 and 5, respectively).
> + *
> + * This function returns the segment selector to utilize as per the conditions
> + * described above. Please note that this functin does not return the value
> + * of the segment selector. The value of the segment selector needs to be
> + * obtained using get_segment_selector and passing the segment selector type
> + * resolved by this function.
> + *
> + * Return: Segment selector to use, among CS, SS, DS, ES, FS or GS.

: negative value when...

> + */
> +static int resolve_seg_selector(struct insn *insn, int regoff, bool get_default)
> +{
> + int i;
> +
> + if (!insn)
> + return -EINVAL;
> +
> + if (get_default)
> + goto default_seg;
> + /*
> + * Check first if we have selector overrides. Having more than
> + * one selector override leads to undefined behavior. We
> + * only use the first one and return

Well, I'd return -EINVAL to catch that undefined behavior. Note in a
local var that I've already seen a seg reg and then if I see another
one, return -EINVAL.

> + */
> + for (i = 0; i < insn->prefixes.nbytes; i++) {
> + switch (insn->prefixes.bytes[i]) {
> + case SEG_CS:
> + return SEG_CS;
> + case SEG_SS:
> + return SEG_SS;
> + case SEG_DS:
> + return SEG_DS;
> + case SEG_ES:
> + return SEG_ES;
> + case SEG_FS:
> + return SEG_FS;
> + case SEG_GS:
> + return SEG_GS;

So what happens if you're in 64-bit mode and you have CS, DS, ES, or SS?
Or is this what @get_default is supposed to do? But it doesn't look like
it, it still returns segments ignored in 64-bit mode.

> + default:
> + return -EINVAL;
> + }
> + }
> +
> +default_seg:
> + /*
> + * If no overrides, use default selectors as described in the
> + * Intel documentation: SS for ESP or EBP. DS for all data references,
> + * except when relative to stack or string destination.
> + * Also, AX, CX and DX are not valid register operands in 16-bit
> + * address encodings.
> + * Callers must interpret the result correctly according to the type
> + * of instructions (e.g., use ES for string instructions).
> + * Also, some values of modrm and sib might seem to indicate the use
> + * of EBP and ESP (e.g., modrm_mod = 0, modrm_rm = 5) but actually
> + * they refer to cases in which only a displacement used. These cases
> + * should be indentified by the caller and not with this function.
> + */
> + switch (regoff) {
> + case offsetof(struct pt_regs, ax):
> + /* fall through */
> + case offsetof(struct pt_regs, cx):
> + /* fall through */
> + case offsetof(struct pt_regs, dx):
> + if (insn && insn->addr_bytes == 2)
> + return -EINVAL;
> + case -EDOM: /* no register involved in address computation */
> + case offsetof(struct pt_regs, bx):
> + /* fall through */
> + case offsetof(struct pt_regs, di):
> + /* fall through */

return SEG_ES;

?

It is even in the comment above. I'm looking at MOVS %es:%rdi, %ds:%rsi,
for example.

> + case offsetof(struct pt_regs, si):
> + return SEG_DS;
> + case offsetof(struct pt_regs, bp):
> + /* fall through */
> + case offsetof(struct pt_regs, sp):
> + return SEG_SS;
> + case offsetof(struct pt_regs, ip):
> + return SEG_CS;
> + default:
> + return -EINVAL;
> + }
> +}
> +
> +/**
> + * get_segment_selector() - obtain segment selector
> + * @regs: Set of registers containing the segment selector
> + * @seg_type: Type of segment selector to obtain
> + * @regoff: Operand offset, in pt_regs, of which the selector is needed

That's gone.

> + *
> + * Obtain the segment selector for any of CS, SS, DS, ES, FS, GS. In
> + * CONFIG_X86_32, the segment is obtained from either pt_regs or
> + * kernel_vm86_regs as applicable. In CONFIG_X86_64, CS and SS are obtained
> + * from pt_regs. DS, ES, FS and GS are obtained by reading the ds and es, fs
> + * and gs, respectively.

... and DS and ES are ignored in long mode.

> + *
> + * Return: Value of the segment selector

... or negative...
> + */
> +static unsigned short get_segment_selector(struct pt_regs *regs,
> + enum segment seg_type)
> +{

--
Regards/Gruss,
Boris.

SUSE Linux GmbH, GF: Felix Imendörffer, Jane Smithard, Graham Norton, HRB 21284 (AG Nürnberg)
--