| Message ID | 20220509194541.GA91598@embeddedor (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | [v2,next] x86/mm/pgtable: Fix -Wstringop-overflow warnings | expand |
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 >
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
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...
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
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
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();
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
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(-)