Re: [PATCH v1 04/19] x86/insn-eval: Handle return values from the decoder
From: Sean Christopherson
Date: Mon Dec 28 2020 - 13:52:21 EST
On Wed, Dec 23, 2020, Borislav Petkov wrote:
> From: Borislav Petkov <bp@xxxxxxx>
>
> 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@xxxxxxx>
> ---
> 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);