| Message ID | 20240828174739.714313-14-debug@rivosinc.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | riscv support for control flow integrity extensions | expand |
On Thu, Aug 29, 2024 at 3:49 AM Deepak Gupta <debug@rivosinc.com> wrote: > > zicfiss protects shadow stack using new page table encodings PTE.W=1, > PTE.R=0 and PTE.X=0. This encoding is reserved if zicfiss is not > implemented or if shadow stack are not enabled. > Loads on shadow stack memory are allowed while stores to shadow stack > memory leads to access faults. Shadow stack accesses to RO memory > leads to store page fault. > > To implement special nature of shadow stack memory where only selected > stores (shadow stack stores from sspush) have to be allowed while rest > of regular stores disallowed, new MMU TLB index is created for shadow > stack. > > Signed-off-by: Deepak Gupta <debug@rivosinc.com> > Suggested-by: Richard Henderson <richard.henderson@linaro.org> > Reviewed-by: Richard Henderson <richard.henderson@linaro.org> > --- > target/riscv/cpu_helper.c | 37 +++++++++++++++++++++++++++++++------ > target/riscv/internals.h | 3 +++ > 2 files changed, 34 insertions(+), 6 deletions(-) > > diff --git a/target/riscv/cpu_helper.c b/target/riscv/cpu_helper.c > index be4ac3d54e..39544cade6 100644 > --- a/target/riscv/cpu_helper.c > +++ b/target/riscv/cpu_helper.c > @@ -893,6 +893,8 @@ static int get_physical_address(CPURISCVState *env, hwaddr *physical, > hwaddr ppn; > int napot_bits = 0; > target_ulong napot_mask; > + bool is_sstack_idx = ((mmu_idx & MMU_IDX_SS_WRITE) == MMU_IDX_SS_WRITE); > + bool sstack_page = false; > > /* > * Check if we should use the background registers for the two > @@ -1101,21 +1103,36 @@ restart: > return TRANSLATE_FAIL; > } > > + target_ulong rwx = pte & (PTE_R | PTE_W | PTE_X); > /* Check for reserved combinations of RWX flags. */ > - switch (pte & (PTE_R | PTE_W | PTE_X)) { > - case PTE_W: > + switch (rwx) { > case PTE_W | PTE_X: > return TRANSLATE_FAIL; > + case PTE_W: > + /* if bcfi enabled, PTE_W is not reserved and shadow stack page */ > + if (cpu_get_bcfien(env) && first_stage) { > + sstack_page = true; > + /* if ss index, read and write allowed. else only read allowed */ > + rwx = is_sstack_idx ? PTE_R | PTE_W : PTE_R; > + break; > + } > + return TRANSLATE_FAIL; > + case PTE_R: > + /* shadow stack writes to readonly memory are page faults */ > + if (is_sstack_idx && access_type == MMU_DATA_STORE) { > + return TRANSLATE_FAIL; > + } > + break; > } > > int prot = 0; > - if (pte & PTE_R) { > + if (rwx & PTE_R) { > prot |= PAGE_READ; > } > - if (pte & PTE_W) { > + if (rwx & PTE_W) { > prot |= PAGE_WRITE; > } > - if (pte & PTE_X) { > + if (rwx & PTE_X) { > bool mxr = false; > > /* > @@ -1160,7 +1177,7 @@ restart: > > if (!((prot >> access_type) & 1)) { > /* Access check failed */ > - return TRANSLATE_FAIL; > + return sstack_page ? TRANSLATE_PMP_FAIL : TRANSLATE_FAIL; Why is it a PMP error if it's a shadow stack page? Alistair
On Thu, Aug 29, 2024 at 09:29:49AM +1000, Alistair Francis wrote: >On Thu, Aug 29, 2024 at 3:49 AM Deepak Gupta <debug@rivosinc.com> wrote: >> >> zicfiss protects shadow stack using new page table encodings PTE.W=1, >> PTE.R=0 and PTE.X=0. This encoding is reserved if zicfiss is not >> implemented or if shadow stack are not enabled. >> Loads on shadow stack memory are allowed while stores to shadow stack >> memory leads to access faults. Shadow stack accesses to RO memory >> leads to store page fault. >> >> To implement special nature of shadow stack memory where only selected >> stores (shadow stack stores from sspush) have to be allowed while rest >> of regular stores disallowed, new MMU TLB index is created for shadow >> stack. >> >> Signed-off-by: Deepak Gupta <debug@rivosinc.com> >> Suggested-by: Richard Henderson <richard.henderson@linaro.org> >> Reviewed-by: Richard Henderson <richard.henderson@linaro.org> >> --- >> target/riscv/cpu_helper.c | 37 +++++++++++++++++++++++++++++++------ >> target/riscv/internals.h | 3 +++ >> 2 files changed, 34 insertions(+), 6 deletions(-) >> >> diff --git a/target/riscv/cpu_helper.c b/target/riscv/cpu_helper.c >> index be4ac3d54e..39544cade6 100644 >> --- a/target/riscv/cpu_helper.c >> +++ b/target/riscv/cpu_helper.c >> @@ -893,6 +893,8 @@ static int get_physical_address(CPURISCVState *env, hwaddr *physical, >> hwaddr ppn; >> int napot_bits = 0; >> target_ulong napot_mask; >> + bool is_sstack_idx = ((mmu_idx & MMU_IDX_SS_WRITE) == MMU_IDX_SS_WRITE); >> + bool sstack_page = false; >> >> /* >> * Check if we should use the background registers for the two >> @@ -1101,21 +1103,36 @@ restart: >> return TRANSLATE_FAIL; >> } >> >> + target_ulong rwx = pte & (PTE_R | PTE_W | PTE_X); >> /* Check for reserved combinations of RWX flags. */ >> - switch (pte & (PTE_R | PTE_W | PTE_X)) { >> - case PTE_W: >> + switch (rwx) { >> case PTE_W | PTE_X: >> return TRANSLATE_FAIL; >> + case PTE_W: >> + /* if bcfi enabled, PTE_W is not reserved and shadow stack page */ >> + if (cpu_get_bcfien(env) && first_stage) { >> + sstack_page = true; >> + /* if ss index, read and write allowed. else only read allowed */ >> + rwx = is_sstack_idx ? PTE_R | PTE_W : PTE_R; >> + break; >> + } >> + return TRANSLATE_FAIL; >> + case PTE_R: >> + /* shadow stack writes to readonly memory are page faults */ >> + if (is_sstack_idx && access_type == MMU_DATA_STORE) { While responding to your question, I noticed there is a bug here. Its a leftover from previous patches where I was promoting shadow stack loads to stores. No need to check `access_type == MMU_DATA_STORE` because we store unwind information as part of tcg compile. Will fix it. >> + return TRANSLATE_FAIL; >> + } >> + break; >> } >> >> int prot = 0; >> - if (pte & PTE_R) { >> + if (rwx & PTE_R) { >> prot |= PAGE_READ; >> } >> - if (pte & PTE_W) { >> + if (rwx & PTE_W) { >> prot |= PAGE_WRITE; >> } >> - if (pte & PTE_X) { >> + if (rwx & PTE_X) { >> bool mxr = false; >> >> /* >> @@ -1160,7 +1177,7 @@ restart: >> >> if (!((prot >> access_type) & 1)) { >> /* Access check failed */ >> - return TRANSLATE_FAIL; >> + return sstack_page ? TRANSLATE_PMP_FAIL : TRANSLATE_FAIL; > >Why is it a PMP error if it's a shadow stack page? A shadow stack page is readable by regular loads. We are making sure of that in `case PTE_W` in above switch case. But shadow stack page is not writeable via regular stores. And must raise access fault. return code `TRANSLATE_PMP_FAIL` is translated to access fault while raising fault. > >Alistair
On Thu, Aug 29, 2024 at 9:45 AM Deepak Gupta <debug@rivosinc.com> wrote: > > On Thu, Aug 29, 2024 at 09:29:49AM +1000, Alistair Francis wrote: > >On Thu, Aug 29, 2024 at 3:49 AM Deepak Gupta <debug@rivosinc.com> wrote: > >> > >> zicfiss protects shadow stack using new page table encodings PTE.W=1, > >> PTE.R=0 and PTE.X=0. This encoding is reserved if zicfiss is not > >> implemented or if shadow stack are not enabled. > >> Loads on shadow stack memory are allowed while stores to shadow stack > >> memory leads to access faults. Shadow stack accesses to RO memory > >> leads to store page fault. > >> > >> To implement special nature of shadow stack memory where only selected > >> stores (shadow stack stores from sspush) have to be allowed while rest > >> of regular stores disallowed, new MMU TLB index is created for shadow > >> stack. > >> > >> Signed-off-by: Deepak Gupta <debug@rivosinc.com> > >> Suggested-by: Richard Henderson <richard.henderson@linaro.org> > >> Reviewed-by: Richard Henderson <richard.henderson@linaro.org> > >> --- > >> target/riscv/cpu_helper.c | 37 +++++++++++++++++++++++++++++++------ > >> target/riscv/internals.h | 3 +++ > >> 2 files changed, 34 insertions(+), 6 deletions(-) > >> > >> diff --git a/target/riscv/cpu_helper.c b/target/riscv/cpu_helper.c > >> index be4ac3d54e..39544cade6 100644 > >> --- a/target/riscv/cpu_helper.c > >> +++ b/target/riscv/cpu_helper.c > >> @@ -893,6 +893,8 @@ static int get_physical_address(CPURISCVState *env, hwaddr *physical, > >> hwaddr ppn; > >> int napot_bits = 0; > >> target_ulong napot_mask; > >> + bool is_sstack_idx = ((mmu_idx & MMU_IDX_SS_WRITE) == MMU_IDX_SS_WRITE); > >> + bool sstack_page = false; > >> > >> /* > >> * Check if we should use the background registers for the two > >> @@ -1101,21 +1103,36 @@ restart: > >> return TRANSLATE_FAIL; > >> } > >> > >> + target_ulong rwx = pte & (PTE_R | PTE_W | PTE_X); > >> /* Check for reserved combinations of RWX flags. */ > >> - switch (pte & (PTE_R | PTE_W | PTE_X)) { > >> - case PTE_W: > >> + switch (rwx) { > >> case PTE_W | PTE_X: > >> return TRANSLATE_FAIL; > >> + case PTE_W: > >> + /* if bcfi enabled, PTE_W is not reserved and shadow stack page */ > >> + if (cpu_get_bcfien(env) && first_stage) { > >> + sstack_page = true; > >> + /* if ss index, read and write allowed. else only read allowed */ > >> + rwx = is_sstack_idx ? PTE_R | PTE_W : PTE_R; > >> + break; > >> + } > >> + return TRANSLATE_FAIL; > >> + case PTE_R: > >> + /* shadow stack writes to readonly memory are page faults */ > >> + if (is_sstack_idx && access_type == MMU_DATA_STORE) { > > While responding to your question, I noticed there is a bug here. Its a leftover from > previous patches where I was promoting shadow stack loads to stores. No need to check > `access_type == MMU_DATA_STORE` because we store unwind information as part of tcg > compile. > > Will fix it. > > >> + return TRANSLATE_FAIL; > >> + } > >> + break; > >> } > >> > >> int prot = 0; > >> - if (pte & PTE_R) { > >> + if (rwx & PTE_R) { > >> prot |= PAGE_READ; > >> } > >> - if (pte & PTE_W) { > >> + if (rwx & PTE_W) { > >> prot |= PAGE_WRITE; > >> } > >> - if (pte & PTE_X) { > >> + if (rwx & PTE_X) { > >> bool mxr = false; > >> > >> /* > >> @@ -1160,7 +1177,7 @@ restart: > >> > >> if (!((prot >> access_type) & 1)) { > >> /* Access check failed */ > >> - return TRANSLATE_FAIL; > >> + return sstack_page ? TRANSLATE_PMP_FAIL : TRANSLATE_FAIL; > > > >Why is it a PMP error if it's a shadow stack page? > > A shadow stack page is readable by regular loads. > We are making sure of that in `case PTE_W` in above switch case. > But shadow stack page is not writeable via regular stores. And must raise > access fault. return code `TRANSLATE_PMP_FAIL` is translated to access fault > while raising fault. Ah, ok. It's worth commenting that we are returning TRANSLATE_PMP_FAIL as that will be translated to an access fault Alistair > > > > >Alistair
On Thu, Aug 29, 2024 at 10:03:04AM +1000, Alistair Francis wrote: >On Thu, Aug 29, 2024 at 9:45 AM Deepak Gupta <debug@rivosinc.com> wrote: >> >> On Thu, Aug 29, 2024 at 09:29:49AM +1000, Alistair Francis wrote: >> >On Thu, Aug 29, 2024 at 3:49 AM Deepak Gupta <debug@rivosinc.com> wrote: >> >> >> >> prot |= PAGE_WRITE; >> >> } >> >> - if (pte & PTE_X) { >> >> + if (rwx & PTE_X) { >> >> bool mxr = false; >> >> >> >> /* >> >> @@ -1160,7 +1177,7 @@ restart: >> >> >> >> if (!((prot >> access_type) & 1)) { >> >> /* Access check failed */ >> >> - return TRANSLATE_FAIL; >> >> + return sstack_page ? TRANSLATE_PMP_FAIL : TRANSLATE_FAIL; >> > >> >Why is it a PMP error if it's a shadow stack page? >> >> A shadow stack page is readable by regular loads. >> We are making sure of that in `case PTE_W` in above switch case. >> But shadow stack page is not writeable via regular stores. And must raise >> access fault. return code `TRANSLATE_PMP_FAIL` is translated to access fault >> while raising fault. > >Ah, ok. It's worth commenting that we are returning TRANSLATE_PMP_FAIL >as that will be translated to an access fault Ack. > >Alistair > >> >> > >> >Alistair
diff --git a/target/riscv/cpu_helper.c b/target/riscv/cpu_helper.c index be4ac3d54e..39544cade6 100644 --- a/target/riscv/cpu_helper.c +++ b/target/riscv/cpu_helper.c @@ -893,6 +893,8 @@ static int get_physical_address(CPURISCVState *env, hwaddr *physical, hwaddr ppn; int napot_bits = 0; target_ulong napot_mask; + bool is_sstack_idx = ((mmu_idx & MMU_IDX_SS_WRITE) == MMU_IDX_SS_WRITE); + bool sstack_page = false; /* * Check if we should use the background registers for the two @@ -1101,21 +1103,36 @@ restart: return TRANSLATE_FAIL; } + target_ulong rwx = pte & (PTE_R | PTE_W | PTE_X); /* Check for reserved combinations of RWX flags. */ - switch (pte & (PTE_R | PTE_W | PTE_X)) { - case PTE_W: + switch (rwx) { case PTE_W | PTE_X: return TRANSLATE_FAIL; + case PTE_W: + /* if bcfi enabled, PTE_W is not reserved and shadow stack page */ + if (cpu_get_bcfien(env) && first_stage) { + sstack_page = true; + /* if ss index, read and write allowed. else only read allowed */ + rwx = is_sstack_idx ? PTE_R | PTE_W : PTE_R; + break; + } + return TRANSLATE_FAIL; + case PTE_R: + /* shadow stack writes to readonly memory are page faults */ + if (is_sstack_idx && access_type == MMU_DATA_STORE) { + return TRANSLATE_FAIL; + } + break; } int prot = 0; - if (pte & PTE_R) { + if (rwx & PTE_R) { prot |= PAGE_READ; } - if (pte & PTE_W) { + if (rwx & PTE_W) { prot |= PAGE_WRITE; } - if (pte & PTE_X) { + if (rwx & PTE_X) { bool mxr = false; /* @@ -1160,7 +1177,7 @@ restart: if (!((prot >> access_type) & 1)) { /* Access check failed */ - return TRANSLATE_FAIL; + return sstack_page ? TRANSLATE_PMP_FAIL : TRANSLATE_FAIL; } target_ulong updated_pte = pte; @@ -1347,9 +1364,17 @@ void riscv_cpu_do_unaligned_access(CPUState *cs, vaddr addr, break; case MMU_DATA_LOAD: cs->exception_index = RISCV_EXCP_LOAD_ADDR_MIS; + /* shadow stack mis aligned accesses are access faults */ + if (mmu_idx & MMU_IDX_SS_WRITE) { + cs->exception_index = RISCV_EXCP_LOAD_ACCESS_FAULT; + } break; case MMU_DATA_STORE: cs->exception_index = RISCV_EXCP_STORE_AMO_ADDR_MIS; + /* shadow stack mis aligned accesses are access faults */ + if (mmu_idx & MMU_IDX_SS_WRITE) { + cs->exception_index = RISCV_EXCP_STORE_AMO_ACCESS_FAULT; + } break; default: g_assert_not_reached(); diff --git a/target/riscv/internals.h b/target/riscv/internals.h index 0ac17bc5ad..ddbdee885b 100644 --- a/target/riscv/internals.h +++ b/target/riscv/internals.h @@ -30,12 +30,15 @@ * - U+2STAGE 0b100 * - S+2STAGE 0b101 * - S+SUM+2STAGE 0b110 + * - Shadow stack+U 0b1000 + * - Shadow stack+S 0b1001 */ #define MMUIdx_U 0 #define MMUIdx_S 1 #define MMUIdx_S_SUM 2 #define MMUIdx_M 3 #define MMU_2STAGE_BIT (1 << 2) +#define MMU_IDX_SS_WRITE (1 << 3) static inline int mmuidx_priv(int mmu_idx) {