Re: [PATCH v9 18/29] x86/insn-eval: Incorporate segment base in linear address computation

Borislav Petkov
2017-10-20T16:09:01Z

On Tue, Oct 03, 2017 at 08:54:21PM -0700, Ricardo Neri wrote:
> insn_get_addr_ref() returns the effective address as defined by the
> section 3.7.5.1 Vol 1 of the Intel 64 and IA-32 Architectures Software
> Developer's Manual. In order to compute the linear address, we must add
> to the effective address the segment base address as set in the segment
> descriptor. The segment descriptor to use depends on the register used as
> operand and segment override prefixes, if any.
>
> In most cases, the segment base address will be 0 if the USER_DS/USER32_DS
> segment is used or if segmentation is not used. However, the base address
> is not necessarily zero if a user programs defines its own segments. This
> is possible by using a local descriptor table.
>
> Since the effective address is a signed quantity, the unsigned segment
> base address is saved in a separate variable and added to the final,
> unsigned, effective address.
>
> 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 | 30 +++++++++++++++++++++++++++---
> 1 file changed, 27 insertions(+), 3 deletions(-)
>
> diff --git a/arch/x86/lib/insn-eval.c b/arch/x86/lib/insn-eval.c
> index dd84819..b3aa891 100644
> --- a/arch/x86/lib/insn-eval.c
> +++ b/arch/x86/lib/insn-eval.c
> @@ -719,8 +719,8 @@ int insn_get_modrm_rm_off(struct insn *insn, struct pt_regs *regs)
> */
> void __user *insn_get_addr_ref(struct insn *insn, struct pt_regs *regs)
> {
> - int addr_offset, base_offset, indx_offset;
> - unsigned long linear_addr = -1L;
> + int addr_offset, base_offset, indx_offset, seg_reg_indx;
> + unsigned long linear_addr = -1L, seg_base_addr;
> long eff_addr, base, indx;
> insn_byte_t sib;
>
> @@ -734,6 +734,14 @@ void __user *insn_get_addr_ref(struct insn *insn, struct pt_regs *regs)
> goto out;
>
> eff_addr = regs_get_register(regs, addr_offset);
> +
> + seg_reg_indx = resolve_seg_reg(insn, regs, addr_offset);
> + if (seg_reg_indx < 0)
> + goto out;
> +
> + seg_base_addr = insn_get_seg_base(regs, seg_reg_indx);
> + if (seg_base_addr == -1L)
> + goto out;

Instead of replicating the same calls three times, add a
get_seg_base_addr() helper and call it where needed.

--
Regards/Gruss,
Boris.

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