[PATCH for-8.1 v2 0/2] target/mips: Avoid shift by negative number in page_table_walk_refill()

[PATCH for-8.1 v2 0/2] target/mips: Avoid shift by negative number in page_table_walk_refill()

[Philippe Mathieu-Daudé]

This is a respin of Peter's patch, but
- pass already-evaluated values to walk_directory() instead
  of an assert(),
- check 'ptew > 1' instead of directory/leaf_shift == -1,
- use unsigned type

Since v1: walk_directory() doesn't have to rely on the to sanitize
          the input values.

Peter, if you don't see this as an improvement I'll take your original
patch.

Regards,

Phil.

Supersedes: <20230717162940.814078-1-peter.maydell@linaro.org>

Philippe Mathieu-Daudé (2):
  target/mips: Pass directory/leaf shift values to walk_directory()
  target/mips: Avoid shift by negative number in
    page_table_walk_refill()

 target/mips/tcg/sysemu/tlb_helper.c | 48 ++++++++++++++---------------
 1 file changed, 24 insertions(+), 24 deletions(-)

-- 
2.38.1

[PATCH for-8.1 v2 1/2] target/mips: Pass directory/leaf shift values to walk_directory()

[Philippe Mathieu-Daudé]

We already evaluated directory_shift and leaf_shift in
page_table_walk_refill(), no need to do that again: pass
as argument.

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
---
 target/mips/tcg/sysemu/tlb_helper.c | 18 ++++++++----------
 1 file changed, 8 insertions(+), 10 deletions(-)

diff --git a/target/mips/tcg/sysemu/tlb_helper.c b/target/mips/tcg/sysemu/tlb_helper.c
index e5e1e9dd3f..e7be649b02 100644
--- a/target/mips/tcg/sysemu/tlb_helper.c
+++ b/target/mips/tcg/sysemu/tlb_helper.c
@@ -623,18 +623,13 @@ static uint64_t get_tlb_entry_layout(CPUMIPSState *env, uint64_t entry,
 
 static int walk_directory(CPUMIPSState *env, uint64_t *vaddr,
         int directory_index, bool *huge_page, bool *hgpg_directory_hit,
-        uint64_t *pw_entrylo0, uint64_t *pw_entrylo1)
+        uint64_t *pw_entrylo0, uint64_t *pw_entrylo1,
+        int directory_shift, int leaf_shift)
 {
     int dph = (env->CP0_PWCtl >> CP0PC_DPH) & 0x1;
     int psn = (env->CP0_PWCtl >> CP0PC_PSN) & 0x3F;
     int hugepg = (env->CP0_PWCtl >> CP0PC_HUGEPG) & 0x1;
     int pf_ptew = (env->CP0_PWField >> CP0PF_PTEW) & 0x3F;
-    int ptew = (env->CP0_PWSize >> CP0PS_PTEW) & 0x3F;
-    int native_shift = (((env->CP0_PWSize >> CP0PS_PS) & 1) == 0) ? 2 : 3;
-    int directory_shift = (ptew > 1) ? -1 :
-            (hugepg && (ptew == 1)) ? native_shift + 1 : native_shift;
-    int leaf_shift = (ptew > 1) ? -1 :
-            (ptew == 1) ? native_shift + 1 : native_shift;
     uint32_t direntry_size = 1 << (directory_shift + 3);
     uint32_t leafentry_size = 1 << (leaf_shift + 3);
     uint64_t entry;
@@ -779,7 +774,8 @@ static bool page_table_walk_refill(CPUMIPSState *env, vaddr address,
     if (gdw > 0) {
         vaddr |= goffset;
         switch (walk_directory(env, &vaddr, pf_gdw, &huge_page, &hgpg_gdhit,
-                               &pw_entrylo0, &pw_entrylo1))
+                               &pw_entrylo0, &pw_entrylo1,
+                               directory_shift, leaf_shift))
         {
         case 0:
             return false;
@@ -795,7 +791,8 @@ static bool page_table_walk_refill(CPUMIPSState *env, vaddr address,
     if (udw > 0) {
         vaddr |= uoffset;
         switch (walk_directory(env, &vaddr, pf_udw, &huge_page, &hgpg_udhit,
-                               &pw_entrylo0, &pw_entrylo1))
+                               &pw_entrylo0, &pw_entrylo1,
+                               directory_shift, leaf_shift))
         {
         case 0:
             return false;
@@ -811,7 +808,8 @@ static bool page_table_walk_refill(CPUMIPSState *env, vaddr address,
     if (mdw > 0) {
         vaddr |= moffset;
         switch (walk_directory(env, &vaddr, pf_mdw, &huge_page, &hgpg_mdhit,
-                               &pw_entrylo0, &pw_entrylo1))
+                               &pw_entrylo0, &pw_entrylo1,
+                               directory_shift, leaf_shift))
         {
         case 0:
             return false;
-- 
2.38.1

Re: [PATCH for-8.1 v2 1/2] target/mips: Pass directory/leaf shift values to walk_directory()

[Peter Maydell]

On Mon, 17 Jul 2023 at 22:36, Philippe Mathieu-Daudé <philmd@linaro.org> wrote:
>
> We already evaluated directory_shift and leaf_shift in
> page_table_walk_refill(), no need to do that again: pass
> as argument.
>
> Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
> ---
>  target/mips/tcg/sysemu/tlb_helper.c | 18 ++++++++----------

Yeah, this is definitely better.

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>

thanks
-- PMM

[PATCH for-8.1 v2 2/2] target/mips: Avoid shift by negative number in page_table_walk_refill()

[Philippe Mathieu-Daudé]

Coverity points out that in page_table_walk_refill() we can shift by
a negative number, which is undefined behaviour (CID 1452918,
1452920, 1452922).  We already catch the negative directory_shift and
leaf_shift as being a "bail out early" case, but not until we've
already used them to calculated some offset values.

Move the calculation of the offset values to after we've done the
"return early if ptew > 1" check.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
[PMD: Check for ptew > 1, use unsigned type]
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
---
 target/mips/tcg/sysemu/tlb_helper.c | 32 +++++++++++++++--------------
 1 file changed, 17 insertions(+), 15 deletions(-)

diff --git a/target/mips/tcg/sysemu/tlb_helper.c b/target/mips/tcg/sysemu/tlb_helper.c
index e7be649b02..7dbc2e24c4 100644
--- a/target/mips/tcg/sysemu/tlb_helper.c
+++ b/target/mips/tcg/sysemu/tlb_helper.c
@@ -624,7 +624,7 @@ static uint64_t get_tlb_entry_layout(CPUMIPSState *env, uint64_t entry,
 static int walk_directory(CPUMIPSState *env, uint64_t *vaddr,
         int directory_index, bool *huge_page, bool *hgpg_directory_hit,
         uint64_t *pw_entrylo0, uint64_t *pw_entrylo1,
-        int directory_shift, int leaf_shift)
+        unsigned directory_shift, unsigned leaf_shift)
 {
     int dph = (env->CP0_PWCtl >> CP0PC_DPH) & 0x1;
     int psn = (env->CP0_PWCtl >> CP0PC_PSN) & 0x3F;
@@ -730,21 +730,11 @@ static bool page_table_walk_refill(CPUMIPSState *env, vaddr address,
 
     /* Other HTW configs */
     int hugepg = (env->CP0_PWCtl >> CP0PC_HUGEPG) & 0x1;
-
-    /* HTW Shift values (depend on entry size) */
-    int directory_shift = (ptew > 1) ? -1 :
-            (hugepg && (ptew == 1)) ? native_shift + 1 : native_shift;
-    int leaf_shift = (ptew > 1) ? -1 :
-            (ptew == 1) ? native_shift + 1 : native_shift;
+    unsigned directory_shift, leaf_shift;
 
     /* Offsets into tables */
-    int goffset = gindex << directory_shift;
-    int uoffset = uindex << directory_shift;
-    int moffset = mindex << directory_shift;
-    int ptoffset0 = (ptindex >> 1) << (leaf_shift + 1);
-    int ptoffset1 = ptoffset0 | (1 << (leaf_shift));
-
-    uint32_t leafentry_size = 1 << (leaf_shift + 3);
+    unsigned goffset, uoffset, moffset, ptoffset0, ptoffset1;
+    uint32_t leafentry_size;
 
     /* Starting address - Page Table Base */
     uint64_t vaddr = env->CP0_PWBase;
@@ -766,10 +756,22 @@ static bool page_table_walk_refill(CPUMIPSState *env, vaddr address,
         /* no structure to walk */
         return false;
     }
-    if ((directory_shift == -1) || (leaf_shift == -1)) {
+    if (ptew > 1) {
         return false;
     }
 
+    /* HTW Shift values (depend on entry size) */
+    directory_shift = (hugepg && (ptew == 1)) ? native_shift + 1 : native_shift;
+    leaf_shift = (ptew == 1) ? native_shift + 1 : native_shift;
+
+    goffset = gindex << directory_shift;
+    uoffset = uindex << directory_shift;
+    moffset = mindex << directory_shift;
+    ptoffset0 = (ptindex >> 1) << (leaf_shift + 1);
+    ptoffset1 = ptoffset0 | (1 << (leaf_shift));
+
+    leafentry_size = 1 << (leaf_shift + 3);
+
     /* Global Directory */
     if (gdw > 0) {
         vaddr |= goffset;
-- 
2.38.1

Re: [PATCH for-8.1 v2 2/2] target/mips: Avoid shift by negative number in page_table_walk_refill()

[Philippe Mathieu-Daudé]

On 17/7/23 23:35, Philippe Mathieu-Daudé wrote:

Oops, invalid authorship, this should be:

From: Peter Maydell <peter.maydell@linaro.org>

> Coverity points out that in page_table_walk_refill() we can shift by
> a negative number, which is undefined behaviour (CID 1452918,
> 1452920, 1452922).  We already catch the negative directory_shift and
> leaf_shift as being a "bail out early" case, but not until we've
> already used them to calculated some offset values.
> 
> Move the calculation of the offset values to after we've done the
> "return early if ptew > 1" check.
> 
> Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
> [PMD: Check for ptew > 1, use unsigned type]
> Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
> ---
>   target/mips/tcg/sysemu/tlb_helper.c | 32 +++++++++++++++--------------
>   1 file changed, 17 insertions(+), 15 deletions(-)

Re: [PATCH for-8.1 v2 2/2] target/mips: Avoid shift by negative number in page_table_walk_refill()

[Peter Maydell]

On Mon, 17 Jul 2023 at 22:35, Philippe Mathieu-Daudé <philmd@linaro.org> wrote:
>
> Coverity points out that in page_table_walk_refill() we can shift by
> a negative number, which is undefined behaviour (CID 1452918,
> 1452920, 1452922).  We already catch the negative directory_shift and
> leaf_shift as being a "bail out early" case, but not until we've
> already used them to calculated some offset values.
>
> Move the calculation of the offset values to after we've done the
> "return early if ptew > 1" check.
>
> Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
> [PMD: Check for ptew > 1, use unsigned type]
> Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>

I think I would expand the commit message a bit, so instead
of the current paragraph 2, something like:

The shifts can be negative only if ptew > 1, so make the
bail-out-early check look directly at that, and only
calculate the shift amounts and the offsets based on them
after we have done that check. This allows
us to simplify the expressions used to calculate the
shift amounts, use an unsigned type, and avoids the
undefined behaviour.


?

Anyway

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>

thanks
-- PMM