Re: [PATCH v1 04/19] x86/insn-eval: Handle return values from the decoder

[Sean Christopherson <seanjc@google.com>]

On Wed, Dec 23, 2020, Borislav Petkov wrote:
> From: Borislav Petkov <bp@suse.de>
> 
> Now that the different instruction-inspecting functions return a value,
> test that and return early from callers if error has been encountered.
>
> While at it, do not call insn_get_modrm() when calling
> insn_get_displacement() because latter will make sure to call
> insn_get_modrm() if ModRM hasn't been parsed yet.
> 
> Signed-off-by: Borislav Petkov <bp@suse.de>
> ---
>  arch/x86/lib/insn-eval.c | 19 +++++++++++++------
>  1 file changed, 13 insertions(+), 6 deletions(-)
> 
> diff --git a/arch/x86/lib/insn-eval.c b/arch/x86/lib/insn-eval.c
> index 265d23a0c334..7e49aaf5454c 100644
> --- a/arch/x86/lib/insn-eval.c
> +++ b/arch/x86/lib/insn-eval.c
> @@ -1106,18 +1106,21 @@ static int get_eff_addr_modrm_16(struct insn *insn, struct pt_regs *regs,
>   * @base_offset will have a register, as an offset from the base of pt_regs,
>   * that can be used to resolve the associated segment.
>   *
> - * -EINVAL on error.
> + * Negative value on error.
>   */
>  static int get_eff_addr_sib(struct insn *insn, struct pt_regs *regs,
>  			    int *base_offset, long *eff_addr)
>  {
>  	long base, indx;
>  	int indx_offset;
> +	int ret;
>  
>  	if (insn->addr_bytes != 8 && insn->addr_bytes != 4)
>  		return -EINVAL;
>  
> -	insn_get_modrm(insn);
> +	ret = insn_get_modrm(insn);

This patch is incomplete/inconsistent, and arguably wrong.

  - get_eff_addr_reg() and get_eff_addr_modrm() still ignore the return of
    insn_get_modrm() after this patch.

  - Calling insn_get_modrm() from get_eff_addr_sib() is unnecessary (unless the
    caller passed uninitialized garbage in @insn) as get_eff_addr_sib() is
    called if and only if sib.nbytes!=0, and sib.nbytes can be non-zero if and
    only if the modrm and sib have been got.

  - get_addr_ref_16() does insn_get_displacement, i.e. guarantees the modrm is
    parsed, while the 32/64 variants do not.

What about adding a prereq patch (or three) to call insn_get_displacement() in
insn_get_addr_ref() prior to switching on insn->addr_bytes?  Then the various
internal helpers could be changed to either omit the sanity checks entirely or
WARN on invalid calls?  Or better yet, add an INSN_WARN_ON() macro that compiles
out the checks by default?  E.g. something like:

diff --git a/arch/x86/lib/insn-eval.c b/arch/x86/lib/insn-eval.c
index 7e49aaf5454c..348969146e0f 100644
--- a/arch/x86/lib/insn-eval.c
+++ b/arch/x86/lib/insn-eval.c
@@ -928,12 +928,8 @@ static int get_seg_base_limit(struct insn *insn, struct pt_regs *regs,
diff --git a/arch/x86/lib/insn-eval.c b/arch/x86/lib/insn-eval.c
index 7e49aaf5454c..348969146e0f 100644
--- a/arch/x86/lib/insn-eval.c
+++ b/arch/x86/lib/insn-eval.c
@@ -928,12 +928,8 @@ static int get_seg_base_limit(struct insn *insn, struct pt_regs *regs,
 static int get_eff_addr_reg(struct insn *insn, struct pt_regs *regs,
                            int *regoff, long *eff_addr)
 {
-       insn_get_modrm(insn);
-
-       if (!insn->modrm.nbytes)
-               return -EINVAL;
-
-       if (X86_MODRM_MOD(insn->modrm.value) != 3)
+       if (INSN_WARN_ON(!insn->modrm.got || !insn->modrm.nbytes ||
+                        X86_MODRM_MOD(insn->modrm.value) != 3)
                return -EINVAL;

        *regoff = get_reg_offset(insn, regs, REG_TYPE_RM);
@@ -978,15 +974,9 @@ static int get_eff_addr_modrm(struct insn *insn, struct pt_regs *regs,
 {
        long tmp;

-       if (insn->addr_bytes != 8 && insn->addr_bytes != 4)
-               return -EINVAL;
-
-       insn_get_modrm(insn);
-
-       if (!insn->modrm.nbytes)
-               return -EINVAL;
-
-       if (X86_MODRM_MOD(insn->modrm.value) > 2)
+       if (INSN_WARN_ON((insn->addr_bytes != 8 && insn->addr_bytes != 4) ||
+                        !insn->modrm.got || !insn->modrm.nbytes ||
+                        X86_MODRM_MOD(insn->modrm.value) > 2)
                return -EINVAL;

        *regoff = get_reg_offset(insn, regs, REG_TYPE_RM);
@@ -1046,15 +1036,8 @@ static int get_eff_addr_modrm_16(struct insn *insn, struct pt_regs *regs,
        int addr_offset1, addr_offset2, ret;
        short addr1 = 0, addr2 = 0, displacement;

-       if (insn->addr_bytes != 2)
-               return -EINVAL;
-
-       insn_get_modrm(insn);
-
-       if (!insn->modrm.nbytes)
-               return -EINVAL;
-
-       if (X86_MODRM_MOD(insn->modrm.value) > 2)
+       if (WARN_ON_ONCE(insn->addr_bytes != 2 || !insn->modrm.got ||
+                        !insn->modrm.nbytes || insn->modrm.value > 2))
                return -EINVAL;

        ret = get_reg_offset_16(insn, regs, &addr_offset1, &addr_offset2);
@@ -1199,10 +1182,7 @@ static void __user *get_addr_ref_16(struct insn *insn, struct pt_regs *regs)
        short eff_addr;
        long tmp;

-       if (insn_get_displacement(insn))
-               goto out;
-
-       if (insn->addr_bytes != 2)
+       if (INSN_WARN_ON(insn->addr_bytes != 2))
                goto out;

        if (X86_MODRM_MOD(insn->modrm.value) == 3) {
@@ -1263,7 +1243,7 @@ static void __user *get_addr_ref_32(struct insn *insn, struct pt_regs *regs)
        long tmp;
        int ret;

-       if (insn->addr_bytes != 4)
+       if (INSN_WARN_ON(insn->addr_bytes != 4))
                goto out;

        if (X86_MODRM_MOD(insn->modrm.value) == 3) {
@@ -1356,7 +1336,7 @@ static void __user *get_addr_ref_64(struct insn *insn, struct pt_regs *regs)
        int regoff, ret;
        long eff_addr;

-       if (insn->addr_bytes != 8)
+       if (INSN_WARN_ON(insn->addr_bytes != 8))
                goto out;

        if (X86_MODRM_MOD(insn->modrm.value) == 3) {
@@ -1408,6 +1388,9 @@ void __user *insn_get_addr_ref(struct insn *insn, struct pt_regs *regs)
        if (!insn || !regs)
                return (void __user *)-1L;

+       if (insn_get_displacement(insn))
+               return (void __user *)-1L;
+
        switch (insn->addr_bytes) {
        case 2:
                return get_addr_ref_16(insn, regs);