| Message ID | 20230807-arm64-gcs-v4-9-68cfa37f9069@kernel.org (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | arm64/gcs: Provide support for GCS in userspace | expand |
On Mon, Aug 07, 2023 at 11:00:14PM +0100, Mark Brown wrote: > diff --git a/arch/arm64/include/asm/pgtable-prot.h b/arch/arm64/include/asm/pgtable-prot.h > index eed814b00a38..b157ae0420ed 100644 > --- a/arch/arm64/include/asm/pgtable-prot.h > +++ b/arch/arm64/include/asm/pgtable-prot.h > @@ -131,15 +131,23 @@ extern bool arm64_use_ng_mappings; > /* 6: PTE_PXN | PTE_WRITE */ > /* 7: PAGE_SHARED_EXEC PTE_PXN | PTE_WRITE | PTE_USER */ > /* 8: PAGE_KERNEL_ROX PTE_UXN */ > -/* 9: PTE_UXN | PTE_USER */ > +/* 9: PAGE_GCS_RO PTE_UXN | PTE_USER */ > /* a: PAGE_KERNEL_EXEC PTE_UXN | PTE_WRITE */ > -/* b: PTE_UXN | PTE_WRITE | PTE_USER */ > +/* b: PAGE_GCS PTE_UXN | PTE_WRITE | PTE_USER */ > /* c: PAGE_KERNEL_RO PTE_UXN | PTE_PXN */ > /* d: PAGE_READONLY PTE_UXN | PTE_PXN | PTE_USER */ > /* e: PAGE_KERNEL PTE_UXN | PTE_PXN | PTE_WRITE */ > /* f: PAGE_SHARED PTE_UXN | PTE_PXN | PTE_WRITE | PTE_USER */ > > +#define _PAGE_GCS (_PAGE_DEFAULT | PTE_UXN | PTE_WRITE | PTE_USER) > +#define _PAGE_GCS_RO (_PAGE_DEFAULT | PTE_UXN | PTE_USER) > + > +#define PAGE_GCS __pgprot(_PAGE_GCS) > +#define PAGE_GCS_RO __pgprot(_PAGE_GCS_RO) > + > #define PIE_E0 ( \ > + PIRx_ELx_PERM(pte_pi_index(_PAGE_GCS), PIE_GCS) | \ > + PIRx_ELx_PERM(pte_pi_index(_PAGE_GCS_RO), PIE_R) | \ > PIRx_ELx_PERM(pte_pi_index(_PAGE_EXECONLY), PIE_X_O) | \ > PIRx_ELx_PERM(pte_pi_index(_PAGE_READONLY_EXEC), PIE_RX) | \ > PIRx_ELx_PERM(pte_pi_index(_PAGE_SHARED_EXEC), PIE_RWX) | \ > @@ -147,6 +155,8 @@ extern bool arm64_use_ng_mappings; > PIRx_ELx_PERM(pte_pi_index(_PAGE_SHARED), PIE_RW)) > > #define PIE_E1 ( \ > + PIRx_ELx_PERM(pte_pi_index(_PAGE_GCS), PIE_RW) | \ > + PIRx_ELx_PERM(pte_pi_index(_PAGE_GCS_RO), PIE_R) | \ Had some thoughts on this. Why do we need the EL1 GCS attributes to map to RW? The instructions we'd use to write the shadow stack are the GCS 'T' variants that run as user already. The only instructions we have in the kernel that would run as EL1 on a user address are the exclusives (futex code or the old deprecated emulation but we don't care about them in this context). So I wonder whether the kernel PIE entry could simply be PIE_NONE_O. Would this be too restrictive for future uses? Given the coherency between a GCS access and a standard data access, we may want to restrict it now until we have a use-case.
On Fri, Aug 11, 2023 at 03:23:12PM +0100, Catalin Marinas wrote: > > #define PIE_E1 ( \ > > + PIRx_ELx_PERM(pte_pi_index(_PAGE_GCS), PIE_RW) | \ > > + PIRx_ELx_PERM(pte_pi_index(_PAGE_GCS_RO), PIE_R) | \ > Had some thoughts on this. Why do we need the EL1 GCS attributes to map > to RW? The instructions we'd use to write the shadow stack are the GCS > 'T' variants that run as user already. > The only instructions we have in the kernel that would run as EL1 on a > user address are the exclusives (futex code or the old deprecated > emulation but we don't care about them in this context). So I wonder > whether the kernel PIE entry could simply be PIE_NONE_O. Would this be > too restrictive for future uses? Given the coherency between a GCS > access and a standard data access, we may want to restrict it now until > we have a use-case. Good point. I remember I originally wrote that before I checked into how things like copying pages for ptrace worked but they don't keep the GCSness of the page so they're fine. I don't think we need to worry about future uses since these are slots reserved for GCS use, if we need a different value later
diff --git a/arch/arm64/include/asm/pgtable-prot.h b/arch/arm64/include/asm/pgtable-prot.h index eed814b00a38..b157ae0420ed 100644 --- a/arch/arm64/include/asm/pgtable-prot.h +++ b/arch/arm64/include/asm/pgtable-prot.h @@ -131,15 +131,23 @@ extern bool arm64_use_ng_mappings; /* 6: PTE_PXN | PTE_WRITE */ /* 7: PAGE_SHARED_EXEC PTE_PXN | PTE_WRITE | PTE_USER */ /* 8: PAGE_KERNEL_ROX PTE_UXN */ -/* 9: PTE_UXN | PTE_USER */ +/* 9: PAGE_GCS_RO PTE_UXN | PTE_USER */ /* a: PAGE_KERNEL_EXEC PTE_UXN | PTE_WRITE */ -/* b: PTE_UXN | PTE_WRITE | PTE_USER */ +/* b: PAGE_GCS PTE_UXN | PTE_WRITE | PTE_USER */ /* c: PAGE_KERNEL_RO PTE_UXN | PTE_PXN */ /* d: PAGE_READONLY PTE_UXN | PTE_PXN | PTE_USER */ /* e: PAGE_KERNEL PTE_UXN | PTE_PXN | PTE_WRITE */ /* f: PAGE_SHARED PTE_UXN | PTE_PXN | PTE_WRITE | PTE_USER */ +#define _PAGE_GCS (_PAGE_DEFAULT | PTE_UXN | PTE_WRITE | PTE_USER) +#define _PAGE_GCS_RO (_PAGE_DEFAULT | PTE_UXN | PTE_USER) + +#define PAGE_GCS __pgprot(_PAGE_GCS) +#define PAGE_GCS_RO __pgprot(_PAGE_GCS_RO) + #define PIE_E0 ( \ + PIRx_ELx_PERM(pte_pi_index(_PAGE_GCS), PIE_GCS) | \ + PIRx_ELx_PERM(pte_pi_index(_PAGE_GCS_RO), PIE_R) | \ PIRx_ELx_PERM(pte_pi_index(_PAGE_EXECONLY), PIE_X_O) | \ PIRx_ELx_PERM(pte_pi_index(_PAGE_READONLY_EXEC), PIE_RX) | \ PIRx_ELx_PERM(pte_pi_index(_PAGE_SHARED_EXEC), PIE_RWX) | \ @@ -147,6 +155,8 @@ extern bool arm64_use_ng_mappings; PIRx_ELx_PERM(pte_pi_index(_PAGE_SHARED), PIE_RW)) #define PIE_E1 ( \ + PIRx_ELx_PERM(pte_pi_index(_PAGE_GCS), PIE_RW) | \ + PIRx_ELx_PERM(pte_pi_index(_PAGE_GCS_RO), PIE_R) | \ PIRx_ELx_PERM(pte_pi_index(_PAGE_EXECONLY), PIE_NONE_O) | \ PIRx_ELx_PERM(pte_pi_index(_PAGE_READONLY_EXEC), PIE_R) | \ PIRx_ELx_PERM(pte_pi_index(_PAGE_SHARED_EXEC), PIE_RW) | \
Pages used for guarded control stacks need to be described to the hardware using the Permission Indirection Extension, GCS is not supported without PIE. In order to support copy on write for guarded stacks we allocate two values, one for active GCSs and one for GCS pages marked as read only prior to copy. Since the actual effect is defined using PIE the specific bit pattern used does not matter to the hardware but we choose two values which differ only in PTE_WRITE in order to help share code with non-PIE cases. Signed-off-by: Mark Brown <broonie@kernel.org> --- arch/arm64/include/asm/pgtable-prot.h | 14 ++++++++++++-- 1 file changed, 12 insertions(+), 2 deletions(-)