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Date: Tue, 9 Aug 2022 11:57:22 +0530 From: Aneesh Kumar K V <aneesh.kumar@...ux.ibm.com> To: Christophe Leroy <christophe.leroy@...roup.eu>, Russell Currey <ruscur@...sell.cc>, "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org> Cc: "mpe@...erman.id.au" <mpe@...erman.id.au>, "ajd@...ux.ibm.com" <ajd@...ux.ibm.com>, "npiggin@...il.com" <npiggin@...il.com>, "anshuman.khandual@....com" <anshuman.khandual@....com>, "linux-hardening@...r.kernel.org" <linux-hardening@...r.kernel.org> Subject: Re: [PATCH v3] powerpc/mm: Support execute-only memory on the Radix MMU On 8/9/22 11:21 AM, Christophe Leroy wrote: > Le 09/08/2022 à 04:44, Russell Currey a écrit : >> The Hash MMU already supports XOM (i.e. mmap with PROT_EXEC only) >> through the execute-only pkey. A PROT_EXEC-only mapping will actually >> map to RX, and then the pkey will be applied on top of it. > > I don't think XOM is a commonly understood accronym. Maybe the first > time you use it it'd be better to say something like: > > The Hash MMU already supports execute-only memory (XOM) > > When you say that Hash MMU supports it through the execute-only pkey, > does it mean that it is taken into account automatically at mmap time, > or does the userspace app has to do something special to use the key ? > If it is the second, it means that depending on whether you are radix or > not, you must do something different ? Is that expected ? > >> >> Radix doesn't have pkeys, but it does have execute permissions built-in >> to the MMU, so all we have to do to support XOM is expose it. >> >> Signed-off-by: Russell Currey <ruscur@...sell.cc> >> --- >> v3: Incorporate Aneesh's suggestions, leave protection_map untouched >> Basic test: https://github.com/ruscur/junkcode/blob/main/mmap_test.c >> >> arch/powerpc/include/asm/book3s/64/pgtable.h | 2 ++ >> arch/powerpc/mm/book3s64/pgtable.c | 11 +++++++++-- >> arch/powerpc/mm/fault.c | 6 +++++- >> 3 files changed, 16 insertions(+), 3 deletions(-) >> >> diff --git a/arch/powerpc/include/asm/book3s/64/pgtable.h b/arch/powerpc/include/asm/book3s/64/pgtable.h >> index 392ff48f77df..486902aff040 100644 >> --- a/arch/powerpc/include/asm/book3s/64/pgtable.h >> +++ b/arch/powerpc/include/asm/book3s/64/pgtable.h >> @@ -151,6 +151,8 @@ >> #define PAGE_COPY_X __pgprot(_PAGE_BASE | _PAGE_READ | _PAGE_EXEC) >> #define PAGE_READONLY __pgprot(_PAGE_BASE | _PAGE_READ) >> #define PAGE_READONLY_X __pgprot(_PAGE_BASE | _PAGE_READ | _PAGE_EXEC) >> +/* Radix only, Hash uses PAGE_READONLY_X + execute-only pkey instead */ >> +#define PAGE_EXECONLY __pgprot(_PAGE_BASE | _PAGE_EXEC) >> >> /* Permission masks used for kernel mappings */ >> #define PAGE_KERNEL __pgprot(_PAGE_BASE | _PAGE_KERNEL_RW) >> diff --git a/arch/powerpc/mm/book3s64/pgtable.c b/arch/powerpc/mm/book3s64/pgtable.c >> index 7b9966402b25..62f63d344596 100644 >> --- a/arch/powerpc/mm/book3s64/pgtable.c >> +++ b/arch/powerpc/mm/book3s64/pgtable.c >> @@ -553,8 +553,15 @@ EXPORT_SYMBOL_GPL(memremap_compat_align); >> >> pgprot_t vm_get_page_prot(unsigned long vm_flags) >> { >> - unsigned long prot = pgprot_val(protection_map[vm_flags & >> - (VM_READ|VM_WRITE|VM_EXEC|VM_SHARED)]); >> + unsigned long prot; >> + >> + /* Radix supports execute-only, but protection_map maps X -> RX */ >> + if (radix_enabled() && ((vm_flags & (VM_READ|VM_WRITE|VM_EXEC)) == VM_EXEC)) { > > Maybe use VM_ACCESS_FLAGS ? > >> + prot = pgprot_val(PAGE_EXECONLY); >> + } else { >> + prot = pgprot_val(protection_map[vm_flags & >> + (VM_READ|VM_WRITE|VM_EXEC|VM_SHARED)]); >> + } >> >> if (vm_flags & VM_SAO) >> prot |= _PAGE_SAO; >> diff --git a/arch/powerpc/mm/fault.c b/arch/powerpc/mm/fault.c >> index 014005428687..59e4cbcf3109 100644 >> --- a/arch/powerpc/mm/fault.c >> +++ b/arch/powerpc/mm/fault.c >> @@ -270,7 +270,11 @@ static bool access_error(bool is_write, bool is_exec, struct vm_area_struct *vma >> return false; >> } >> >> - if (unlikely(!vma_is_accessible(vma))) >> + /* On Radix, a read fault could be from PROT_NONE or PROT_EXEC */ >> + if (unlikely(radix_enabled() && !(vma->vm_flags & VM_READ))) >> + return true; > > Why do you need the radix_enabled() here ? > Even if it doesn't fault directly, reading a non readable area is still > an error and should be handled as such, even on hardware that will not > generate a fault for it at the first place. So I'd just do: > > if (!(vma->vm_flags & VM_READ))) > return true; > >> + /* Check for a PROT_NONE fault on other MMUs */ >> + else if (unlikely(!vma_is_accessible(vma))) >> return true; >> /* >> * We should ideally do the vma pkey access check here. But in the > > Don't use an if/else construct, there is no other 'else' in that > function, or in similar functions like bad_kernel_fault() for instance. > > So leave the !vma_is_accessible(vma) untouched and add your check as a > standalone check before or after it. What does vma_is_accessible() check bring if we have the VM_READ check unconditional ? -aneesh
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