[PATCH v2][next] x86/mm/pgtable: Fix -Wstringop-overflow warnings

[PATCH v2][next] x86/mm/pgtable: Fix -Wstringop-overflow warnings

[Gustavo A. R. Silva]

Fix the following -Wstringop-overflow warnings when building with GCC-12.1:

arch/x86/mm/pgtable.c:437:13: warning: 'preallocate_pmds' accessing 8 bytes in a region of size 0 [-Wstringop-overflow=]
arch/x86/mm/pgtable.c:440:13: warning: 'preallocate_pmds' accessing 8 bytes in a region of size 0 [-Wstringop-overflow=]
arch/x86/mm/pgtable.c:462:9: warning: 'free_pmds' accessing 8 bytes in a region of size 0 [-Wstringop-overflow=]
arch/x86/mm/pgtable.c:454:9: warning: 'pgd_prepopulate_pmd' accessing 8 bytes in a region of size 0 [-Wstringop-overflow=]
arch/x86/mm/pgtable.c:455:9: warning: 'pgd_prepopulate_user_pmd' accessing 8 bytes in a region of size 0 [-Wstringop-overflow=]
arch/x86/mm/pgtable.c:464:9: warning: 'free_pmds' accessing 8 bytes in a region of size 0 [-Wstringop-overflow=]

There is a case in which PREALLOCATED_PMDS, MAX_PREALLOCATED_PMDS,
PREALLOCATED_USER_PMDS and MAX_PREALLOCATED_USER_PMDS are defined as
zero:

204 #else  /* !CONFIG_X86_PAE */
205 
206 /* No need to prepopulate any pagetable entries in non-PAE modes. */
207 #define PREALLOCATED_PMDS       0
208 #define MAX_PREALLOCATED_PMDS   0
209 #define PREALLOCATED_USER_PMDS   0
210 #define MAX_PREALLOCATED_USER_PMDS 0
211 #endif  /* CONFIG_X86_PAE */

It seems that GCC is legitimately complaining about the fact that, under
certain circumstances, u_pmds and pmds are declared as zero-length arrays
in the stack and, of course, they are not flexible arrays.

424 pgd_t *pgd_alloc(struct mm_struct *mm)
425 {
426         pgd_t *pgd;
427         pmd_t *u_pmds[MAX_PREALLOCATED_USER_PMDS];
428         pmd_t *pmds[MAX_PREALLOCATED_PMDS];
429

Notice that "Accessing elements of zero-length arrays declared in such
contexts is undefined and may be diagnosed."[1]

We can fix this by checking that MAX_PREALLOCATED_PMDS and MAX_PREALLOCATED_USER_PMDS
are different than zero, prior to passing u_pmds amd pmds as arguments to any
function, in this case to functions preallocate_pmds(), pgd_prepopulate_pmd()
and free_pmds().

This helps with the ongoing efforts to globally enable
-Wstringop-overflow.

[1] https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html

Link: https://github.com/KSPP/linux/issues/181
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
---
Changes in v2:
 - Check MAX_PREALLOCATED_PMDS and MAX_PREALLOCATED_USER_PMDS
   instead of using pointer notation.
   Link: https://lore.kernel.org/linux-hardening/20220401005834.GA182932@embeddedor/
 - Update changelog text.

 arch/x86/mm/pgtable.c | 16 ++++++++++------
 1 file changed, 10 insertions(+), 6 deletions(-)

diff --git a/arch/x86/mm/pgtable.c b/arch/x86/mm/pgtable.c
index f16059e9a85e..96c3f402a1da 100644
--- a/arch/x86/mm/pgtable.c
+++ b/arch/x86/mm/pgtable.c
@@ -434,14 +434,18 @@ pgd_t *pgd_alloc(struct mm_struct *mm)
 
 	mm->pgd = pgd;
 
-	if (preallocate_pmds(mm, pmds, PREALLOCATED_PMDS) != 0)
-		goto out_free_pgd;
+	if (MAX_PREALLOCATED_PMDS != 0 && MAX_PREALLOCATED_USER_PMDS != 0) {
+		if (preallocate_pmds(mm, pmds, PREALLOCATED_PMDS) != 0)
+			goto out_free_pgd;
 
-	if (preallocate_pmds(mm, u_pmds, PREALLOCATED_USER_PMDS) != 0)
-		goto out_free_pmds;
+		if (preallocate_pmds(mm, u_pmds, PREALLOCATED_USER_PMDS) != 0)
+			goto out_free_pmds;
 
-	if (paravirt_pgd_alloc(mm) != 0)
-		goto out_free_user_pmds;
+		if (paravirt_pgd_alloc(mm) != 0)
+			goto out_free_user_pmds;
+	} else {
+		goto out_free_pgd;
+	}
 
 	/*
 	 * Make sure that pre-populating the pmds is atomic with
-- 
2.27.0

Re: [PATCH v2][next] x86/mm/pgtable: Fix -Wstringop-overflow warnings

[Kees Cook]

On Mon, May 09, 2022 at 02:45:41PM -0500, Gustavo A. R. Silva wrote:
> Fix the following -Wstringop-overflow warnings when building with GCC-12.1:
> 
> arch/x86/mm/pgtable.c:437:13: warning: 'preallocate_pmds' accessing 8 bytes in a region of size 0 [-Wstringop-overflow=]
> arch/x86/mm/pgtable.c:440:13: warning: 'preallocate_pmds' accessing 8 bytes in a region of size 0 [-Wstringop-overflow=]
> arch/x86/mm/pgtable.c:462:9: warning: 'free_pmds' accessing 8 bytes in a region of size 0 [-Wstringop-overflow=]
> arch/x86/mm/pgtable.c:454:9: warning: 'pgd_prepopulate_pmd' accessing 8 bytes in a region of size 0 [-Wstringop-overflow=]
> arch/x86/mm/pgtable.c:455:9: warning: 'pgd_prepopulate_user_pmd' accessing 8 bytes in a region of size 0 [-Wstringop-overflow=]
> arch/x86/mm/pgtable.c:464:9: warning: 'free_pmds' accessing 8 bytes in a region of size 0 [-Wstringop-overflow=]
> 
> There is a case in which PREALLOCATED_PMDS, MAX_PREALLOCATED_PMDS,
> PREALLOCATED_USER_PMDS and MAX_PREALLOCATED_USER_PMDS are defined as
> zero:
> 
> 204 #else  /* !CONFIG_X86_PAE */
> 205 
> 206 /* No need to prepopulate any pagetable entries in non-PAE modes. */
> 207 #define PREALLOCATED_PMDS       0
> 208 #define MAX_PREALLOCATED_PMDS   0
> 209 #define PREALLOCATED_USER_PMDS   0
> 210 #define MAX_PREALLOCATED_USER_PMDS 0
> 211 #endif  /* CONFIG_X86_PAE */
> 
> It seems that GCC is legitimately complaining about the fact that, under
> certain circumstances, u_pmds and pmds are declared as zero-length arrays
> in the stack and, of course, they are not flexible arrays.

Ah yeah, I've run into this a few times. Since the relationship between
the macro pairs can't be seen by GCC, it gets upset (i.e. sizeof(u_pmds)
has no relationship wtih PREALLOCATED_USER_PMDS and the calls weren't
inlined, so it can't see that it'll always be 0 and 0).

> 424 pgd_t *pgd_alloc(struct mm_struct *mm)
> 425 {
> 426         pgd_t *pgd;
> 427         pmd_t *u_pmds[MAX_PREALLOCATED_USER_PMDS];
> 428         pmd_t *pmds[MAX_PREALLOCATED_PMDS];
> 429
> 
> Notice that "Accessing elements of zero-length arrays declared in such
> contexts is undefined and may be diagnosed."[1]
> 
> We can fix this by checking that MAX_PREALLOCATED_PMDS and MAX_PREALLOCATED_USER_PMDS
> are different than zero, prior to passing u_pmds amd pmds as arguments to any
> function, in this case to functions preallocate_pmds(), pgd_prepopulate_pmd()
> and free_pmds().
> 
> This helps with the ongoing efforts to globally enable
> -Wstringop-overflow.
> 
> [1] https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html
> 
> Link: https://github.com/KSPP/linux/issues/181
> Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
> ---
> Changes in v2:
>  - Check MAX_PREALLOCATED_PMDS and MAX_PREALLOCATED_USER_PMDS
>    instead of using pointer notation.
>    Link: https://lore.kernel.org/linux-hardening/20220401005834.GA182932@embeddedor/
>  - Update changelog text.
> 
>  arch/x86/mm/pgtable.c | 16 ++++++++++------
>  1 file changed, 10 insertions(+), 6 deletions(-)
> 
> diff --git a/arch/x86/mm/pgtable.c b/arch/x86/mm/pgtable.c
> index f16059e9a85e..96c3f402a1da 100644
> --- a/arch/x86/mm/pgtable.c
> +++ b/arch/x86/mm/pgtable.c
> @@ -434,14 +434,18 @@ pgd_t *pgd_alloc(struct mm_struct *mm)
>  
>  	mm->pgd = pgd;
>  
> -	if (preallocate_pmds(mm, pmds, PREALLOCATED_PMDS) != 0)
> -		goto out_free_pgd;
> +	if (MAX_PREALLOCATED_PMDS != 0 && MAX_PREALLOCATED_USER_PMDS != 0) {
> +		if (preallocate_pmds(mm, pmds, PREALLOCATED_PMDS) != 0)
> +			goto out_free_pgd;
>  
> -	if (preallocate_pmds(mm, u_pmds, PREALLOCATED_USER_PMDS) != 0)
> -		goto out_free_pmds;
> +		if (preallocate_pmds(mm, u_pmds, PREALLOCATED_USER_PMDS) != 0)
> +			goto out_free_pmds;
>  
> -	if (paravirt_pgd_alloc(mm) != 0)
> -		goto out_free_user_pmds;
> +		if (paravirt_pgd_alloc(mm) != 0)
> +			goto out_free_user_pmds;
> +	} else {
> +		goto out_free_pgd;

The "all 0" case shouldn't be a failure mode; it should just skip the
preallocate_pmds() calls.

> +	}
>  
>  	/*
>  	 * Make sure that pre-populating the pmds is atomic with
> -- 
> 2.27.0
> 

-- 
Kees Cook

Re: [PATCH v2][next] x86/mm/pgtable: Fix -Wstringop-overflow warnings

[Gustavo A. R. Silva]

On Mon, May 09, 2022 at 12:59:15PM -0700, Kees Cook wrote:
> On Mon, May 09, 2022 at 02:45:41PM -0500, Gustavo A. R. Silva wrote:
> > Fix the following -Wstringop-overflow warnings when building with GCC-12.1:
> > 
> > arch/x86/mm/pgtable.c:437:13: warning: 'preallocate_pmds' accessing 8 bytes in a region of size 0 [-Wstringop-overflow=]
> > arch/x86/mm/pgtable.c:440:13: warning: 'preallocate_pmds' accessing 8 bytes in a region of size 0 [-Wstringop-overflow=]
> > arch/x86/mm/pgtable.c:462:9: warning: 'free_pmds' accessing 8 bytes in a region of size 0 [-Wstringop-overflow=]
> > arch/x86/mm/pgtable.c:454:9: warning: 'pgd_prepopulate_pmd' accessing 8 bytes in a region of size 0 [-Wstringop-overflow=]
> > arch/x86/mm/pgtable.c:455:9: warning: 'pgd_prepopulate_user_pmd' accessing 8 bytes in a region of size 0 [-Wstringop-overflow=]
> > arch/x86/mm/pgtable.c:464:9: warning: 'free_pmds' accessing 8 bytes in a region of size 0 [-Wstringop-overflow=]
> > 
> > There is a case in which PREALLOCATED_PMDS, MAX_PREALLOCATED_PMDS,
> > PREALLOCATED_USER_PMDS and MAX_PREALLOCATED_USER_PMDS are defined as
> > zero:
> > 
> > 204 #else  /* !CONFIG_X86_PAE */
> > 205 
> > 206 /* No need to prepopulate any pagetable entries in non-PAE modes. */
> > 207 #define PREALLOCATED_PMDS       0
> > 208 #define MAX_PREALLOCATED_PMDS   0
> > 209 #define PREALLOCATED_USER_PMDS   0
> > 210 #define MAX_PREALLOCATED_USER_PMDS 0
> > 211 #endif  /* CONFIG_X86_PAE */
> > 
> > It seems that GCC is legitimately complaining about the fact that, under
> > certain circumstances, u_pmds and pmds are declared as zero-length arrays
> > in the stack and, of course, they are not flexible arrays.
> 
> Ah yeah, I've run into this a few times. Since the relationship between
> the macro pairs can't be seen by GCC, it gets upset (i.e. sizeof(u_pmds)
> has no relationship wtih PREALLOCATED_USER_PMDS and the calls weren't
> inlined, so it can't see that it'll always be 0 and 0).
> 
> > 424 pgd_t *pgd_alloc(struct mm_struct *mm)
> > 425 {
> > 426         pgd_t *pgd;
> > 427         pmd_t *u_pmds[MAX_PREALLOCATED_USER_PMDS];
> > 428         pmd_t *pmds[MAX_PREALLOCATED_PMDS];
> > 429
> > 
> > Notice that "Accessing elements of zero-length arrays declared in such
> > contexts is undefined and may be diagnosed."[1]
> > 
> > We can fix this by checking that MAX_PREALLOCATED_PMDS and MAX_PREALLOCATED_USER_PMDS
> > are different than zero, prior to passing u_pmds amd pmds as arguments to any
> > function, in this case to functions preallocate_pmds(), pgd_prepopulate_pmd()
> > and free_pmds().
> > 
> > This helps with the ongoing efforts to globally enable
> > -Wstringop-overflow.
> > 
> > [1] https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html
> > 
> > Link: https://github.com/KSPP/linux/issues/181
> > Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
> > ---
> > Changes in v2:
> >  - Check MAX_PREALLOCATED_PMDS and MAX_PREALLOCATED_USER_PMDS
> >    instead of using pointer notation.
> >    Link: https://lore.kernel.org/linux-hardening/20220401005834.GA182932@embeddedor/
> >  - Update changelog text.
> > 
> >  arch/x86/mm/pgtable.c | 16 ++++++++++------
> >  1 file changed, 10 insertions(+), 6 deletions(-)
> > 
> > diff --git a/arch/x86/mm/pgtable.c b/arch/x86/mm/pgtable.c
> > index f16059e9a85e..96c3f402a1da 100644
> > --- a/arch/x86/mm/pgtable.c
> > +++ b/arch/x86/mm/pgtable.c
> > @@ -434,14 +434,18 @@ pgd_t *pgd_alloc(struct mm_struct *mm)
> >  
> >  	mm->pgd = pgd;
> >  
> > -	if (preallocate_pmds(mm, pmds, PREALLOCATED_PMDS) != 0)
> > -		goto out_free_pgd;
> > +	if (MAX_PREALLOCATED_PMDS != 0 && MAX_PREALLOCATED_USER_PMDS != 0) {
> > +		if (preallocate_pmds(mm, pmds, PREALLOCATED_PMDS) != 0)
> > +			goto out_free_pgd;
> >  
> > -	if (preallocate_pmds(mm, u_pmds, PREALLOCATED_USER_PMDS) != 0)
> > -		goto out_free_pmds;
> > +		if (preallocate_pmds(mm, u_pmds, PREALLOCATED_USER_PMDS) != 0)
> > +			goto out_free_pmds;
> >  
> > -	if (paravirt_pgd_alloc(mm) != 0)
> > -		goto out_free_user_pmds;
> > +		if (paravirt_pgd_alloc(mm) != 0)
> > +			goto out_free_user_pmds;
> > +	} else {
> > +		goto out_free_pgd;
> 
> The "all 0" case shouldn't be a failure mode; it should just skip the
> preallocate_pmds() calls.

Do you mean something like this:

diff --git a/arch/x86/mm/pgtable.c b/arch/x86/mm/pgtable.c
index f16059e9a85e..4dae168408f1 100644
--- a/arch/x86/mm/pgtable.c
+++ b/arch/x86/mm/pgtable.c
@@ -434,11 +434,13 @@ pgd_t *pgd_alloc(struct mm_struct *mm)

        mm->pgd = pgd;

-       if (preallocate_pmds(mm, pmds, PREALLOCATED_PMDS) != 0)
-               goto out_free_pgd;
+       if (MAX_PREALLOCATED_PMDS != 0 && MAX_PREALLOCATED_USER_PMDS != 0) {
+               if (preallocate_pmds(mm, pmds, PREALLOCATED_PMDS) != 0)
+                       goto out_free_pgd;

-       if (preallocate_pmds(mm, u_pmds, PREALLOCATED_USER_PMDS) != 0)
-               goto out_free_pmds;
+               if (preallocate_pmds(mm, u_pmds, PREALLOCATED_USER_PMDS) != 0)
+                       goto out_free_pmds;
+       }

        if (paravirt_pgd_alloc(mm) != 0)
                goto out_free_user_pmds;

It seems that the above is not enough, because we have the same issue
when calling pgd_prepopulate_pmd(), pgd_prepopulate_user_pmd() and
free_pmds():

  CC      arch/x86/mm/pgtable.o
arch/x86/mm/pgtable.c: In function 'pgd_alloc':
arch/x86/mm/pgtable.c:464:9: warning: 'free_pmds' accessing 8 bytes in a region of size 0 [-Wstringop-overflow=]
  464 |         free_pmds(mm, u_pmds, PREALLOCATED_USER_PMDS);
      |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
arch/x86/mm/pgtable.c:464:9: note: referencing argument 2 of type 'pmd_t *[0]'
arch/x86/mm/pgtable.c:213:13: note: in a call to function 'free_pmds'
  213 | static void free_pmds(struct mm_struct *mm, pmd_t *pmds[], int count)
      |             ^~~~~~~~~
arch/x86/mm/pgtable.c:466:9: warning: 'free_pmds' accessing 8 bytes in a region of size 0 [-Wstringop-overflow=]
  466 |         free_pmds(mm, pmds, PREALLOCATED_PMDS);
      |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
arch/x86/mm/pgtable.c:466:9: note: referencing argument 2 of type 'pmd_t *[0]'
arch/x86/mm/pgtable.c:213:13: note: in a call to function 'free_pmds'
  213 | static void free_pmds(struct mm_struct *mm, pmd_t *pmds[], int count)
      |             ^~~~~~~~~
arch/x86/mm/pgtable.c:456:9: warning: 'pgd_prepopulate_pmd' accessing 8 bytes in a region of size 0 [-Wstringop-overflow=]
  456 |         pgd_prepopulate_pmd(mm, pgd, pmds);
      |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
arch/x86/mm/pgtable.c:456:9: note: referencing argument 3 of type 'pmd_t *[0]'
arch/x86/mm/pgtable.c:296:13: note: in a call to function 'pgd_prepopulate_pmd'
  296 | static void pgd_prepopulate_pmd(struct mm_struct *mm, pgd_t *pgd, pmd_t *pmds[])
      |             ^~~~~~~~~~~~~~~~~~~
arch/x86/mm/pgtable.c:457:9: warning: 'pgd_prepopulate_user_pmd' accessing 8 bytes in a region of size 0 [-Wstringop-overflow=]
  457 |         pgd_prepopulate_user_pmd(mm, pgd, u_pmds);
      |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
arch/x86/mm/pgtable.c:457:9: note: referencing argument 3 of type 'pmd_t *[0]'
arch/x86/mm/pgtable.c:320:13: note: in a call to function 'pgd_prepopulate_user_pmd'
  320 | static void pgd_prepopulate_user_pmd(struct mm_struct *mm,
      |             ^~~~~~~~~~~~~~~~~~~~~~~~

--
Gustavo
> 
> > +	}
> >  
> >  	/*
> >  	 * Make sure that pre-populating the pmds is atomic with
> > -- 
> > 2.27.0
> > 
> 
> -- 
> Kees Cook

Re: [PATCH v2][next] x86/mm/pgtable: Fix -Wstringop-overflow warnings

[Kees Cook]

On Mon, May 09, 2022 at 03:50:56PM -0500, Gustavo A. R. Silva wrote:
> On Mon, May 09, 2022 at 12:59:15PM -0700, Kees Cook wrote:
> > On Mon, May 09, 2022 at 02:45:41PM -0500, Gustavo A. R. Silva wrote:
> > > Fix the following -Wstringop-overflow warnings when building with GCC-12.1:
> > > 
> > > arch/x86/mm/pgtable.c:437:13: warning: 'preallocate_pmds' accessing 8 bytes in a region of size 0 [-Wstringop-overflow=]
> > > arch/x86/mm/pgtable.c:440:13: warning: 'preallocate_pmds' accessing 8 bytes in a region of size 0 [-Wstringop-overflow=]
> > > arch/x86/mm/pgtable.c:462:9: warning: 'free_pmds' accessing 8 bytes in a region of size 0 [-Wstringop-overflow=]
> > > arch/x86/mm/pgtable.c:454:9: warning: 'pgd_prepopulate_pmd' accessing 8 bytes in a region of size 0 [-Wstringop-overflow=]
> > > arch/x86/mm/pgtable.c:455:9: warning: 'pgd_prepopulate_user_pmd' accessing 8 bytes in a region of size 0 [-Wstringop-overflow=]
> > > arch/x86/mm/pgtable.c:464:9: warning: 'free_pmds' accessing 8 bytes in a region of size 0 [-Wstringop-overflow=]
> > > 
> > > There is a case in which PREALLOCATED_PMDS, MAX_PREALLOCATED_PMDS,
> > > PREALLOCATED_USER_PMDS and MAX_PREALLOCATED_USER_PMDS are defined as
> > > zero:
> > > 
> > > 204 #else  /* !CONFIG_X86_PAE */
> > > 205 
> > > 206 /* No need to prepopulate any pagetable entries in non-PAE modes. */
> > > 207 #define PREALLOCATED_PMDS       0
> > > 208 #define MAX_PREALLOCATED_PMDS   0
> > > 209 #define PREALLOCATED_USER_PMDS   0
> > > 210 #define MAX_PREALLOCATED_USER_PMDS 0
> > > 211 #endif  /* CONFIG_X86_PAE */
> > > 
> > > It seems that GCC is legitimately complaining about the fact that, under
> > > certain circumstances, u_pmds and pmds are declared as zero-length arrays
> > > in the stack and, of course, they are not flexible arrays.
> > 
> > Ah yeah, I've run into this a few times. Since the relationship between
> > the macro pairs can't be seen by GCC, it gets upset (i.e. sizeof(u_pmds)
> > has no relationship wtih PREALLOCATED_USER_PMDS and the calls weren't
> > inlined, so it can't see that it'll always be 0 and 0).
> > 
> > > 424 pgd_t *pgd_alloc(struct mm_struct *mm)
> > > 425 {
> > > 426         pgd_t *pgd;
> > > 427         pmd_t *u_pmds[MAX_PREALLOCATED_USER_PMDS];
> > > 428         pmd_t *pmds[MAX_PREALLOCATED_PMDS];
> > > 429
> > > 
> > > Notice that "Accessing elements of zero-length arrays declared in such
> > > contexts is undefined and may be diagnosed."[1]
> > > 
> > > We can fix this by checking that MAX_PREALLOCATED_PMDS and MAX_PREALLOCATED_USER_PMDS
> > > are different than zero, prior to passing u_pmds amd pmds as arguments to any
> > > function, in this case to functions preallocate_pmds(), pgd_prepopulate_pmd()
> > > and free_pmds().
> > > 
> > > This helps with the ongoing efforts to globally enable
> > > -Wstringop-overflow.
> > > 
> > > [1] https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html
> > > 
> > > Link: https://github.com/KSPP/linux/issues/181
> > > Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
> > > ---
> > > Changes in v2:
> > >  - Check MAX_PREALLOCATED_PMDS and MAX_PREALLOCATED_USER_PMDS
> > >    instead of using pointer notation.
> > >    Link: https://lore.kernel.org/linux-hardening/20220401005834.GA182932@embeddedor/
> > >  - Update changelog text.
> > > 
> > >  arch/x86/mm/pgtable.c | 16 ++++++++++------
> > >  1 file changed, 10 insertions(+), 6 deletions(-)
> > > 
> > > diff --git a/arch/x86/mm/pgtable.c b/arch/x86/mm/pgtable.c
> > > index f16059e9a85e..96c3f402a1da 100644
> > > --- a/arch/x86/mm/pgtable.c
> > > +++ b/arch/x86/mm/pgtable.c
> > > @@ -434,14 +434,18 @@ pgd_t *pgd_alloc(struct mm_struct *mm)
> > >  
> > >  	mm->pgd = pgd;
> > >  
> > > -	if (preallocate_pmds(mm, pmds, PREALLOCATED_PMDS) != 0)
> > > -		goto out_free_pgd;
> > > +	if (MAX_PREALLOCATED_PMDS != 0 && MAX_PREALLOCATED_USER_PMDS != 0) {
> > > +		if (preallocate_pmds(mm, pmds, PREALLOCATED_PMDS) != 0)
> > > +			goto out_free_pgd;
> > >  
> > > -	if (preallocate_pmds(mm, u_pmds, PREALLOCATED_USER_PMDS) != 0)
> > > -		goto out_free_pmds;
> > > +		if (preallocate_pmds(mm, u_pmds, PREALLOCATED_USER_PMDS) != 0)
> > > +			goto out_free_pmds;
> > >  
> > > -	if (paravirt_pgd_alloc(mm) != 0)
> > > -		goto out_free_user_pmds;
> > > +		if (paravirt_pgd_alloc(mm) != 0)
> > > +			goto out_free_user_pmds;
> > > +	} else {
> > > +		goto out_free_pgd;
> > 
> > The "all 0" case shouldn't be a failure mode; it should just skip the
> > preallocate_pmds() calls.
> 
> Do you mean something like this:
> 
> diff --git a/arch/x86/mm/pgtable.c b/arch/x86/mm/pgtable.c
> index f16059e9a85e..4dae168408f1 100644
> --- a/arch/x86/mm/pgtable.c
> +++ b/arch/x86/mm/pgtable.c
> @@ -434,11 +434,13 @@ pgd_t *pgd_alloc(struct mm_struct *mm)
> 
>         mm->pgd = pgd;
> 
> -       if (preallocate_pmds(mm, pmds, PREALLOCATED_PMDS) != 0)
> -               goto out_free_pgd;
> +       if (MAX_PREALLOCATED_PMDS != 0 && MAX_PREALLOCATED_USER_PMDS != 0) {
> +               if (preallocate_pmds(mm, pmds, PREALLOCATED_PMDS) != 0)
> +                       goto out_free_pgd;
> 
> -       if (preallocate_pmds(mm, u_pmds, PREALLOCATED_USER_PMDS) != 0)
> -               goto out_free_pmds;
> +               if (preallocate_pmds(mm, u_pmds, PREALLOCATED_USER_PMDS) != 0)
> +                       goto out_free_pmds;
> +       }
> 
>         if (paravirt_pgd_alloc(mm) != 0)
>                 goto out_free_user_pmds;
> 
> It seems that the above is not enough, because we have the same issue
> when calling pgd_prepopulate_pmd(), pgd_prepopulate_user_pmd() and
> free_pmds():
> 
>   CC      arch/x86/mm/pgtable.o
> arch/x86/mm/pgtable.c: In function 'pgd_alloc':
> arch/x86/mm/pgtable.c:464:9: warning: 'free_pmds' accessing 8 bytes in a region of size 0 [-Wstringop-overflow=]
>   464 |         free_pmds(mm, u_pmds, PREALLOCATED_USER_PMDS);
>       |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Ugh. Perhaps just marking both preallocate_pmds() and free_pmds() as
inline is enough to let the compiler "see" everything correctly?

Otherwise, they'll likely each need the same check that was added to
pgd_prepopulate_pmd() ages ago for a similar situation...

-- 
Kees Cook

Re: [PATCH v2][next] x86/mm/pgtable: Fix -Wstringop-overflow warnings

[Gustavo A. R. Silva]

On Mon, May 09, 2022 at 01:54:32PM -0700, Kees Cook wrote:
> On Mon, May 09, 2022 at 03:50:56PM -0500, Gustavo A. R. Silva wrote:
> > On Mon, May 09, 2022 at 12:59:15PM -0700, Kees Cook wrote:
> > > On Mon, May 09, 2022 at 02:45:41PM -0500, Gustavo A. R. Silva wrote:
> > > > Fix the following -Wstringop-overflow warnings when building with GCC-12.1:
> > > > 
> > > > arch/x86/mm/pgtable.c:437:13: warning: 'preallocate_pmds' accessing 8 bytes in a region of size 0 [-Wstringop-overflow=]
> > > > arch/x86/mm/pgtable.c:440:13: warning: 'preallocate_pmds' accessing 8 bytes in a region of size 0 [-Wstringop-overflow=]
> > > > arch/x86/mm/pgtable.c:462:9: warning: 'free_pmds' accessing 8 bytes in a region of size 0 [-Wstringop-overflow=]
> > > > arch/x86/mm/pgtable.c:454:9: warning: 'pgd_prepopulate_pmd' accessing 8 bytes in a region of size 0 [-Wstringop-overflow=]
> > > > arch/x86/mm/pgtable.c:455:9: warning: 'pgd_prepopulate_user_pmd' accessing 8 bytes in a region of size 0 [-Wstringop-overflow=]
> > > > arch/x86/mm/pgtable.c:464:9: warning: 'free_pmds' accessing 8 bytes in a region of size 0 [-Wstringop-overflow=]
> > > > 
> > > > There is a case in which PREALLOCATED_PMDS, MAX_PREALLOCATED_PMDS,
> > > > PREALLOCATED_USER_PMDS and MAX_PREALLOCATED_USER_PMDS are defined as
> > > > zero:
> > > > 
> > > > 204 #else  /* !CONFIG_X86_PAE */
> > > > 205 
> > > > 206 /* No need to prepopulate any pagetable entries in non-PAE modes. */
> > > > 207 #define PREALLOCATED_PMDS       0
> > > > 208 #define MAX_PREALLOCATED_PMDS   0
> > > > 209 #define PREALLOCATED_USER_PMDS   0
> > > > 210 #define MAX_PREALLOCATED_USER_PMDS 0
> > > > 211 #endif  /* CONFIG_X86_PAE */
> > > > 
> > > > It seems that GCC is legitimately complaining about the fact that, under
> > > > certain circumstances, u_pmds and pmds are declared as zero-length arrays
> > > > in the stack and, of course, they are not flexible arrays.
> > > 
> > > Ah yeah, I've run into this a few times. Since the relationship between
> > > the macro pairs can't be seen by GCC, it gets upset (i.e. sizeof(u_pmds)
> > > has no relationship wtih PREALLOCATED_USER_PMDS and the calls weren't
> > > inlined, so it can't see that it'll always be 0 and 0).
> > > 
> > > > 424 pgd_t *pgd_alloc(struct mm_struct *mm)
> > > > 425 {
> > > > 426         pgd_t *pgd;
> > > > 427         pmd_t *u_pmds[MAX_PREALLOCATED_USER_PMDS];
> > > > 428         pmd_t *pmds[MAX_PREALLOCATED_PMDS];
> > > > 429
> > > > 
> > > > Notice that "Accessing elements of zero-length arrays declared in such
> > > > contexts is undefined and may be diagnosed."[1]
> > > > 
> > > > We can fix this by checking that MAX_PREALLOCATED_PMDS and MAX_PREALLOCATED_USER_PMDS
> > > > are different than zero, prior to passing u_pmds amd pmds as arguments to any
> > > > function, in this case to functions preallocate_pmds(), pgd_prepopulate_pmd()
> > > > and free_pmds().
> > > > 
> > > > This helps with the ongoing efforts to globally enable
> > > > -Wstringop-overflow.
> > > > 
> > > > [1] https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html
> > > > 
> > > > Link: https://github.com/KSPP/linux/issues/181
> > > > Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
> > > > ---
> > > > Changes in v2:
> > > >  - Check MAX_PREALLOCATED_PMDS and MAX_PREALLOCATED_USER_PMDS
> > > >    instead of using pointer notation.
> > > >    Link: https://lore.kernel.org/linux-hardening/20220401005834.GA182932@embeddedor/
> > > >  - Update changelog text.
> > > > 
> > > >  arch/x86/mm/pgtable.c | 16 ++++++++++------
> > > >  1 file changed, 10 insertions(+), 6 deletions(-)
> > > > 
> > > > diff --git a/arch/x86/mm/pgtable.c b/arch/x86/mm/pgtable.c
> > > > index f16059e9a85e..96c3f402a1da 100644
> > > > --- a/arch/x86/mm/pgtable.c
> > > > +++ b/arch/x86/mm/pgtable.c
> > > > @@ -434,14 +434,18 @@ pgd_t *pgd_alloc(struct mm_struct *mm)
> > > >  
> > > >  	mm->pgd = pgd;
> > > >  
> > > > -	if (preallocate_pmds(mm, pmds, PREALLOCATED_PMDS) != 0)
> > > > -		goto out_free_pgd;
> > > > +	if (MAX_PREALLOCATED_PMDS != 0 && MAX_PREALLOCATED_USER_PMDS != 0) {
> > > > +		if (preallocate_pmds(mm, pmds, PREALLOCATED_PMDS) != 0)
> > > > +			goto out_free_pgd;
> > > >  
> > > > -	if (preallocate_pmds(mm, u_pmds, PREALLOCATED_USER_PMDS) != 0)
> > > > -		goto out_free_pmds;
> > > > +		if (preallocate_pmds(mm, u_pmds, PREALLOCATED_USER_PMDS) != 0)
> > > > +			goto out_free_pmds;
> > > >  
> > > > -	if (paravirt_pgd_alloc(mm) != 0)
> > > > -		goto out_free_user_pmds;
> > > > +		if (paravirt_pgd_alloc(mm) != 0)
> > > > +			goto out_free_user_pmds;
> > > > +	} else {
> > > > +		goto out_free_pgd;
> > > 
> > > The "all 0" case shouldn't be a failure mode; it should just skip the
> > > preallocate_pmds() calls.
> > 
> > Do you mean something like this:
> > 
> > diff --git a/arch/x86/mm/pgtable.c b/arch/x86/mm/pgtable.c
> > index f16059e9a85e..4dae168408f1 100644
> > --- a/arch/x86/mm/pgtable.c
> > +++ b/arch/x86/mm/pgtable.c
> > @@ -434,11 +434,13 @@ pgd_t *pgd_alloc(struct mm_struct *mm)
> > 
> >         mm->pgd = pgd;
> > 
> > -       if (preallocate_pmds(mm, pmds, PREALLOCATED_PMDS) != 0)
> > -               goto out_free_pgd;
> > +       if (MAX_PREALLOCATED_PMDS != 0 && MAX_PREALLOCATED_USER_PMDS != 0) {
> > +               if (preallocate_pmds(mm, pmds, PREALLOCATED_PMDS) != 0)
> > +                       goto out_free_pgd;
> > 
> > -       if (preallocate_pmds(mm, u_pmds, PREALLOCATED_USER_PMDS) != 0)
> > -               goto out_free_pmds;
> > +               if (preallocate_pmds(mm, u_pmds, PREALLOCATED_USER_PMDS) != 0)
> > +                       goto out_free_pmds;
> > +       }
> > 
> >         if (paravirt_pgd_alloc(mm) != 0)
> >                 goto out_free_user_pmds;
> > 
> > It seems that the above is not enough, because we have the same issue
> > when calling pgd_prepopulate_pmd(), pgd_prepopulate_user_pmd() and
> > free_pmds():
> > 
> >   CC      arch/x86/mm/pgtable.o
> > arch/x86/mm/pgtable.c: In function 'pgd_alloc':
> > arch/x86/mm/pgtable.c:464:9: warning: 'free_pmds' accessing 8 bytes in a region of size 0 [-Wstringop-overflow=]
> >   464 |         free_pmds(mm, u_pmds, PREALLOCATED_USER_PMDS);
> >       |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> 
> Ugh. Perhaps just marking both preallocate_pmds() and free_pmds() as
> inline is enough to let the compiler "see" everything correctly?

It doesn't seem to work... however, the following piece of code implies
that pmds and u_pmds should be first preallocated through preallocate_pmds(),
which cannot happen if (MAX_PREALLOCATED_PMDS != 0 && MAX_PREALLOCATED_USER_PMDS != 0)

448         /*
449          * Make sure that pre-populating the pmds is atomic with
450          * respect to anything walking the pgd_list, so that they
451          * never see a partially populated pgd.
452          */
453         spin_lock(&pgd_lock);
454
455         pgd_ctor(mm, pgd);
456         pgd_prepopulate_pmd(mm, pgd, pmds);
457         pgd_prepopulate_user_pmd(mm, pgd, u_pmds);
458
459         spin_unlock(&pgd_lock);
460
461         return pgd;

So, my question here is why do you think the "all 0" case should only skip the
preallocate_pmds() calls and not the pgd_prepopulate_pmd() calls too?

> 
> Otherwise, they'll likely each need the same check that was added to
> pgd_prepopulate_pmd() ages ago for a similar situation...

uhm... that doesn't seem to have an impact nowadays, or at least now
Wstringop-overflow sees the problem first, because now the issue is
detected at the moment of passing the arguments to the the function
and not when actually executing the function?

otherwise, I think we wouldn't see this error:

arch/x86/mm/pgtable.c:454:9: warning: 'pgd_prepopulate_pmd' accessing 8 bytes in a region of size 0 [-Wstringop-overflow=]
  454 |         pgd_prepopulate_pmd(mm, pgd, pmds);
      |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
arch/x86/mm/pgtable.c:454:9: note: referencing argument 3 of type 'pmd_t *[0]'
arch/x86/mm/pgtable.c:296:13: note: in a call to function 'pgd_prepopulate_pmd'
  296 | static void pgd_prepopulate_pmd(struct mm_struct *mm, pgd_t *pgd, pmd_t *pmds[])
      |             ^~~~~~~~~~~~~~~~~~~


Thanks
--
Gustavo

Re: [PATCH v2][next] x86/mm/pgtable: Fix -Wstringop-overflow warnings

[Gustavo A. R. Silva]

On Tue, May 10, 2022 at 09:12:02AM -0500, Gustavo A. R. Silva wrote:
> > > > > --- a/arch/x86/mm/pgtable.c
> > > > > +++ b/arch/x86/mm/pgtable.c
> > > > > @@ -434,14 +434,18 @@ pgd_t *pgd_alloc(struct mm_struct *mm)
> > > > >  
> > > > >  	mm->pgd = pgd;
> > > > >  
> > > > > -	if (preallocate_pmds(mm, pmds, PREALLOCATED_PMDS) != 0)
> > > > > -		goto out_free_pgd;
> > > > > +	if (MAX_PREALLOCATED_PMDS != 0 && MAX_PREALLOCATED_USER_PMDS != 0) {
> > > > > +		if (preallocate_pmds(mm, pmds, PREALLOCATED_PMDS) != 0)
> > > > > +			goto out_free_pgd;
> > > > >  
> > > > > -	if (preallocate_pmds(mm, u_pmds, PREALLOCATED_USER_PMDS) != 0)
> > > > > -		goto out_free_pmds;
> > > > > +		if (preallocate_pmds(mm, u_pmds, PREALLOCATED_USER_PMDS) != 0)
> > > > > +			goto out_free_pmds;
> > > > >  
> > > > > -	if (paravirt_pgd_alloc(mm) != 0)
> > > > > -		goto out_free_user_pmds;
> > > > > +		if (paravirt_pgd_alloc(mm) != 0)
> > > > > +			goto out_free_user_pmds;
> > > > > +	} else {
> > > > > +		goto out_free_pgd;
> > > > 
> > > > The "all 0" case shouldn't be a failure mode; it should just skip the
> > > > preallocate_pmds() calls.
> > > 
> > > Do you mean something like this:
> > > 
> > > diff --git a/arch/x86/mm/pgtable.c b/arch/x86/mm/pgtable.c
> > > index f16059e9a85e..4dae168408f1 100644
> > > --- a/arch/x86/mm/pgtable.c
> > > +++ b/arch/x86/mm/pgtable.c
> > > @@ -434,11 +434,13 @@ pgd_t *pgd_alloc(struct mm_struct *mm)
> > > 
> > >         mm->pgd = pgd;
> > > 
> > > -       if (preallocate_pmds(mm, pmds, PREALLOCATED_PMDS) != 0)
> > > -               goto out_free_pgd;
> > > +       if (MAX_PREALLOCATED_PMDS != 0 && MAX_PREALLOCATED_USER_PMDS != 0) {
> > > +               if (preallocate_pmds(mm, pmds, PREALLOCATED_PMDS) != 0)
> > > +                       goto out_free_pgd;
> > > 
> > > -       if (preallocate_pmds(mm, u_pmds, PREALLOCATED_USER_PMDS) != 0)
> > > -               goto out_free_pmds;
> > > +               if (preallocate_pmds(mm, u_pmds, PREALLOCATED_USER_PMDS) != 0)
> > > +                       goto out_free_pmds;
> > > +       }
> > > 
> > >         if (paravirt_pgd_alloc(mm) != 0)
> > >                 goto out_free_user_pmds;
> > > 
> > > It seems that the above is not enough, because we have the same issue
> > > when calling pgd_prepopulate_pmd(), pgd_prepopulate_user_pmd() and
> > > free_pmds():
> > > 
> > >   CC      arch/x86/mm/pgtable.o
> > > arch/x86/mm/pgtable.c: In function 'pgd_alloc':
> > > arch/x86/mm/pgtable.c:464:9: warning: 'free_pmds' accessing 8 bytes in a region of size 0 [-Wstringop-overflow=]
> > >   464 |         free_pmds(mm, u_pmds, PREALLOCATED_USER_PMDS);
> > >       |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> > 
> > Ugh. Perhaps just marking both preallocate_pmds() and free_pmds() as
> > inline is enough to let the compiler "see" everything correctly?
> 
> It doesn't seem to work... however, the following piece of code implies
> that pmds and u_pmds should be first preallocated through preallocate_pmds(),
> which cannot happen if (MAX_PREALLOCATED_PMDS != 0 && MAX_PREALLOCATED_USER_PMDS != 0)

I wanted to say: which cannot happen if MAX_PREALLOCATED_PMDS == 0 && MAX_PREALLOCATED_USER_PMDS == 0

> 
> 448         /*
> 449          * Make sure that pre-populating the pmds is atomic with
> 450          * respect to anything walking the pgd_list, so that they
> 451          * never see a partially populated pgd.
> 452          */
> 453         spin_lock(&pgd_lock);
> 454
> 455         pgd_ctor(mm, pgd);
> 456         pgd_prepopulate_pmd(mm, pgd, pmds);
> 457         pgd_prepopulate_user_pmd(mm, pgd, u_pmds);
> 458
> 459         spin_unlock(&pgd_lock);
> 460
> 461         return pgd;
> 
> So, my question here is why do you think the "all 0" case should only skip the
> preallocate_pmds() calls and not the pgd_prepopulate_pmd() calls too?
> 
> > 
> > Otherwise, they'll likely each need the same check that was added to
> > pgd_prepopulate_pmd() ages ago for a similar situation...
> 
> uhm... that doesn't seem to have an impact nowadays, or at least now
> Wstringop-overflow sees the problem first, because now the issue is
> detected at the moment of passing the arguments to the the function
> and not when actually executing the function?
> 
> otherwise, I think we wouldn't see this error:
> 
> arch/x86/mm/pgtable.c:454:9: warning: 'pgd_prepopulate_pmd' accessing 8 bytes in a region of size 0 [-Wstringop-overflow=]
>   454 |         pgd_prepopulate_pmd(mm, pgd, pmds);
>       |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> arch/x86/mm/pgtable.c:454:9: note: referencing argument 3 of type 'pmd_t *[0]'
> arch/x86/mm/pgtable.c:296:13: note: in a call to function 'pgd_prepopulate_pmd'
>   296 | static void pgd_prepopulate_pmd(struct mm_struct *mm, pgd_t *pgd, pmd_t *pmds[])
>       |             ^~~~~~~~~~~~~~~~~~~
> 

Thanks
--
Gustavo

Re: [PATCH v2][next] x86/mm/pgtable: Fix -Wstringop-overflow warnings

[Kees Cook]

On Tue, May 10, 2022 at 09:12:02AM -0500, Gustavo A. R. Silva wrote:
> It doesn't seem to work... however, the following piece of code implies
> that pmds and u_pmds should be first preallocated through preallocate_pmds(),
> which cannot happen if (MAX_PREALLOCATED_PMDS != 0 && MAX_PREALLOCATED_USER_PMDS != 0)

This works, weirdly:

diff --git a/arch/x86/mm/pgtable.c b/arch/x86/mm/pgtable.c
index 3481b35cb4ec..937a87b404c3 100644
--- a/arch/x86/mm/pgtable.c
+++ b/arch/x86/mm/pgtable.c
@@ -424,8 +424,8 @@ static inline void _pgd_free(pgd_t *pgd)
 pgd_t *pgd_alloc(struct mm_struct *mm)
 {
 	pgd_t *pgd;
-	pmd_t *u_pmds[MAX_PREALLOCATED_USER_PMDS];
-	pmd_t *pmds[MAX_PREALLOCATED_PMDS];
+	pmd_t *u_pmds[MAX_PREALLOCATED_USER_PMDS + 1];
+	pmd_t *pmds[MAX_PREALLOCATED_PMDS + 1];
 
 	pgd = _pgd_alloc();
 

-- 
Kees Cook