| Message ID | 20241008-v5_user_cfi_series-v6-16-60d9fe073f37@rivosinc.com (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | riscv control-flow integrity for usermode | expand |
On Tue, 2024-10-08 at 15:36 -0700, Deepak Gupta wrote: > +unsigned long shstk_alloc_thread_stack(struct task_struct *tsk, > + const struct kernel_clone_args *args) > +{ > + unsigned long addr, size; > + > + /* If shadow stack is not supported, return 0 */ > + if (!cpu_supports_shadow_stack()) > + return 0; > + > + /* > + * If shadow stack is not enabled on the new thread, skip any > + * switch to a new shadow stack. > + */ > + if (!is_shstk_enabled(tsk)) > + return 0; > + > + /* > + * For CLONE_VFORK the child will share the parents shadow stack. > + * Set base = 0 and size = 0, this is special means to track this state > + * so the freeing logic run for child knows to leave it alone. > + */ > + if (args->flags & CLONE_VFORK) { > + set_shstk_base(tsk, 0, 0); > + return 0; > + } > + > + /* > + * For !CLONE_VM the child will use a copy of the parents shadow > + * stack. > + */ > + if (!(args->flags & CLONE_VM)) > + return 0; > + > + /* > + * reaching here means, CLONE_VM was specified and thus a separate shadow > + * stack is needed for new cloned thread. Note: below allocation is happening > + * using current mm. > + */ > + size = calc_shstk_size(args->stack_size); > + addr = allocate_shadow_stack(0, size, 0, false); > + if (IS_ERR_VALUE(addr)) > + return addr; > + > + set_shstk_base(tsk, addr, size); > + > + return addr + size; > +} A lot of this patch and the previous one is similar to x86's and arm's. It great that we can have consistency around this behavior. There might be enough consistency to refactor some of the arch code into a kernel/shstk.c. Should we try?
On Tue, Oct 08, 2024 at 10:55:29PM +0000, Edgecombe, Rick P wrote: >On Tue, 2024-10-08 at 15:36 -0700, Deepak Gupta wrote: >> +unsigned long shstk_alloc_thread_stack(struct task_struct *tsk, >> + const struct kernel_clone_args *args) >> +{ >> + unsigned long addr, size; >> + >> + /* If shadow stack is not supported, return 0 */ >> + if (!cpu_supports_shadow_stack()) >> + return 0; >> + >> + /* >> + * If shadow stack is not enabled on the new thread, skip any >> + * switch to a new shadow stack. >> + */ >> + if (!is_shstk_enabled(tsk)) >> + return 0; >> + >> + /* >> + * For CLONE_VFORK the child will share the parents shadow stack. >> + * Set base = 0 and size = 0, this is special means to track this state >> + * so the freeing logic run for child knows to leave it alone. >> + */ >> + if (args->flags & CLONE_VFORK) { >> + set_shstk_base(tsk, 0, 0); >> + return 0; >> + } >> + >> + /* >> + * For !CLONE_VM the child will use a copy of the parents shadow >> + * stack. >> + */ >> + if (!(args->flags & CLONE_VM)) >> + return 0; >> + >> + /* >> + * reaching here means, CLONE_VM was specified and thus a separate shadow >> + * stack is needed for new cloned thread. Note: below allocation is happening >> + * using current mm. >> + */ >> + size = calc_shstk_size(args->stack_size); >> + addr = allocate_shadow_stack(0, size, 0, false); >> + if (IS_ERR_VALUE(addr)) >> + return addr; >> + >> + set_shstk_base(tsk, addr, size); >> + >> + return addr + size; >> +} > >A lot of this patch and the previous one is similar to x86's and arm's. It great >that we can have consistency around this behavior. > >There might be enough consistency to refactor some of the arch code into a >kernel/shstk.c. > >Should we try? Yeah you're right. Honestly, I've been shameless in adapting most of the flows from x86 `shstk.c` for risc-v. So thank you for that. Now that we've `ARCH_HAS_USER_SHADOW_STACK` part of multiple patch series (riscv shadowstack, clone3 and I think arm64 gcs series as well). It's probably the appropriate time to find common grounds. This is what I suggest - move most of the common/arch agnostic shadow stack stuff in kernel/shstk.c This gets part of compile if `ARCH_HAS_USER_SHADOW_STACK` is enabled/selected. - allow arch specific branch out guard checks for "if cpu supports", "is shadow stack enabled on the task_struct" (I expect each arch layout of task_struct will be different, no point finding common ground there), etc. I think it's worth a try. If you already don't have patches, I'll spend some time to see what it takes to converge in my next version. If I end up into some roadblock, will use this thread for further discussion.
On Tue, 2024-10-08 at 16:17 -0700, Deepak Gupta wrote: > Yeah you're right. Honestly, I've been shameless in adapting most of the flows > from x86 `shstk.c` for risc-v. So thank you for that. All good, glad we ended up with similar behavior. > > Now that we've `ARCH_HAS_USER_SHADOW_STACK` part of multiple patch series (riscv > shadowstack, clone3 and I think arm64 gcs series as well). It's probably the > appropriate time to find common grounds. There have been bugs in the similar bits of code. So will be nice to not have to fix them in each arch too. > > This is what I suggest > > - move most of the common/arch agnostic shadow stack stuff in kernel/shstk.c > This gets part of compile if `ARCH_HAS_USER_SHADOW_STACK` is enabled/selected. Yea, I guess we have commonality for (in x86 naming): - map_shadow_stack() - shstk_free() - shstk_alloc_thread_stack() - shstk_setup() The signal part starts to diverge. Then I guess x86 has a different prctl interface. > > - allow arch specific branch out guard checks for "if cpu supports", "is shadow stack > enabled on the task_struct" (I expect each arch layout of task_struct will be > different, no point finding common ground there), etc. Sure. > > I think it's worth a try. > If you already don't have patches, I'll spend some time to see what it takes to > converge in my next version. If I end up into some roadblock, will use this thread > for further discussion. Sounds good. I have not looked at it too much.
On Tue, Oct 08, 2024 at 10:55:29PM +0000, Edgecombe, Rick P wrote: > A lot of this patch and the previous one is similar to x86's and arm's. It great > that we can have consistency around this behavior. > There might be enough consistency to refactor some of the arch code into a > kernel/shstk.c. > Should we try? I think so - I think we discussed it before. I was thinking of looking at it once the clone3() stuff settles down, I don't want to trigger any unneeded refectorings there and cause further delays.
diff --git a/arch/riscv/include/asm/usercfi.h b/arch/riscv/include/asm/usercfi.h index 4fa201b4fc4e..4da9cbc8f9b5 100644 --- a/arch/riscv/include/asm/usercfi.h +++ b/arch/riscv/include/asm/usercfi.h @@ -8,6 +8,9 @@ #ifndef __ASSEMBLY__ #include <linux/types.h> +struct task_struct; +struct kernel_clone_args; + #ifdef CONFIG_RISCV_USER_CFI struct cfi_status { unsigned long ubcfi_en : 1; /* Enable for backward cfi. */ @@ -17,6 +20,28 @@ struct cfi_status { unsigned long shdw_stk_size; /* size of shadow stack */ }; +unsigned long shstk_alloc_thread_stack(struct task_struct *tsk, + const struct kernel_clone_args *args); +void shstk_release(struct task_struct *tsk); +void set_shstk_base(struct task_struct *task, unsigned long shstk_addr, unsigned long size); +unsigned long get_shstk_base(struct task_struct *task, unsigned long *size); +void set_active_shstk(struct task_struct *task, unsigned long shstk_addr); +bool is_shstk_enabled(struct task_struct *task); + +#else + +#define shstk_alloc_thread_stack(tsk, args) 0 + +#define shstk_release(tsk) + +#define get_shstk_base(task, size) 0UL + +#define set_shstk_base(task, shstk_addr, size) + +#define set_active_shstk(task, shstk_addr) + +#define is_shstk_enabled(task) false + #endif /* CONFIG_RISCV_USER_CFI */ #endif /* __ASSEMBLY__ */ diff --git a/arch/riscv/kernel/process.c b/arch/riscv/kernel/process.c index 1f2574fb2edb..f6f58b1ed905 100644 --- a/arch/riscv/kernel/process.c +++ b/arch/riscv/kernel/process.c @@ -28,6 +28,7 @@ #include <asm/vector.h> #include <asm/cpufeature.h> #include <asm/exec.h> +#include <asm/usercfi.h> #if defined(CONFIG_STACKPROTECTOR) && !defined(CONFIG_STACKPROTECTOR_PER_TASK) #include <linux/stackprotector.h> @@ -203,7 +204,7 @@ int arch_dup_task_struct(struct task_struct *dst, struct task_struct *src) void exit_thread(struct task_struct *tsk) { - + shstk_release(tsk); } int copy_thread(struct task_struct *p, const struct kernel_clone_args *args) @@ -211,6 +212,7 @@ int copy_thread(struct task_struct *p, const struct kernel_clone_args *args) unsigned long clone_flags = args->flags; unsigned long usp = args->stack; unsigned long tls = args->tls; + unsigned long ssp = 0; struct pt_regs *childregs = task_pt_regs(p); memset(&p->thread.s, 0, sizeof(p->thread.s)); @@ -225,11 +227,18 @@ int copy_thread(struct task_struct *p, const struct kernel_clone_args *args) p->thread.s[0] = (unsigned long)args->fn; p->thread.s[1] = (unsigned long)args->fn_arg; } else { + /* allocate new shadow stack if needed. In case of CLONE_VM we have to */ + ssp = shstk_alloc_thread_stack(p, args); + if (IS_ERR_VALUE(ssp)) + return PTR_ERR((void *)ssp); + *childregs = *(current_pt_regs()); /* Turn off status.VS */ riscv_v_vstate_off(childregs); if (usp) /* User fork */ childregs->sp = usp; + /* if needed, set new ssp */ + ssp ? set_active_shstk(p, ssp) : 0; if (clone_flags & CLONE_SETTLS) childregs->tp = tls; childregs->a0 = 0; /* Return value of fork() */ diff --git a/arch/riscv/kernel/usercfi.c b/arch/riscv/kernel/usercfi.c index 96bb324abafb..6cd166b73316 100644 --- a/arch/riscv/kernel/usercfi.c +++ b/arch/riscv/kernel/usercfi.c @@ -19,6 +19,41 @@ #define SHSTK_ENTRY_SIZE sizeof(void *) +bool is_shstk_enabled(struct task_struct *task) +{ + return task->thread_info.user_cfi_state.ubcfi_en ? true : false; +} + +void set_shstk_base(struct task_struct *task, unsigned long shstk_addr, unsigned long size) +{ + task->thread_info.user_cfi_state.shdw_stk_base = shstk_addr; + task->thread_info.user_cfi_state.shdw_stk_size = size; +} + +unsigned long get_shstk_base(struct task_struct *task, unsigned long *size) +{ + if (size) + *size = task->thread_info.user_cfi_state.shdw_stk_size; + return task->thread_info.user_cfi_state.shdw_stk_base; +} + +void set_active_shstk(struct task_struct *task, unsigned long shstk_addr) +{ + task->thread_info.user_cfi_state.user_shdw_stk = shstk_addr; +} + +/* + * If size is 0, then to be compatible with regular stack we want it to be as big as + * regular stack. Else PAGE_ALIGN it and return back + */ +static unsigned long calc_shstk_size(unsigned long size) +{ + if (size) + return PAGE_ALIGN(size); + + return PAGE_ALIGN(min_t(unsigned long long, rlimit(RLIMIT_STACK), SZ_4G)); +} + /* * Writes on shadow stack can either be `sspush` or `ssamoswap`. `sspush` can happen * implicitly on current shadow stack pointed to by CSR_SSP. `ssamoswap` takes pointer to @@ -143,3 +178,89 @@ SYSCALL_DEFINE3(map_shadow_stack, unsigned long, addr, unsigned long, size, unsi return allocate_shadow_stack(addr, aligned_size, size, set_tok); } + +/* + * This gets called during clone/clone3/fork. And is needed to allocate a shadow stack for + * cases where CLONE_VM is specified and thus a different stack is specified by user. We + * thus need a separate shadow stack too. How does separate shadow stack is specified by + * user is still being debated. Once that's settled, remove this part of the comment. + * This function simply returns 0 if shadow stack are not supported or if separate shadow + * stack allocation is not needed (like in case of !CLONE_VM) + */ +unsigned long shstk_alloc_thread_stack(struct task_struct *tsk, + const struct kernel_clone_args *args) +{ + unsigned long addr, size; + + /* If shadow stack is not supported, return 0 */ + if (!cpu_supports_shadow_stack()) + return 0; + + /* + * If shadow stack is not enabled on the new thread, skip any + * switch to a new shadow stack. + */ + if (!is_shstk_enabled(tsk)) + return 0; + + /* + * For CLONE_VFORK the child will share the parents shadow stack. + * Set base = 0 and size = 0, this is special means to track this state + * so the freeing logic run for child knows to leave it alone. + */ + if (args->flags & CLONE_VFORK) { + set_shstk_base(tsk, 0, 0); + return 0; + } + + /* + * For !CLONE_VM the child will use a copy of the parents shadow + * stack. + */ + if (!(args->flags & CLONE_VM)) + return 0; + + /* + * reaching here means, CLONE_VM was specified and thus a separate shadow + * stack is needed for new cloned thread. Note: below allocation is happening + * using current mm. + */ + size = calc_shstk_size(args->stack_size); + addr = allocate_shadow_stack(0, size, 0, false); + if (IS_ERR_VALUE(addr)) + return addr; + + set_shstk_base(tsk, addr, size); + + return addr + size; +} + +void shstk_release(struct task_struct *tsk) +{ + unsigned long base = 0, size = 0; + /* If shadow stack is not supported or not enabled, nothing to release */ + if (!cpu_supports_shadow_stack() || + !is_shstk_enabled(tsk)) + return; + + /* + * When fork() with CLONE_VM fails, the child (tsk) already has a + * shadow stack allocated, and exit_thread() calls this function to + * free it. In this case the parent (current) and the child share + * the same mm struct. Move forward only when they're same. + */ + if (!tsk->mm || tsk->mm != current->mm) + return; + + /* + * We know shadow stack is enabled but if base is NULL, then + * this task is not managing its own shadow stack (CLONE_VFORK). So + * skip freeing it. + */ + base = get_shstk_base(tsk, &size); + if (!base) + return; + + vm_munmap(base, size); + set_shstk_base(tsk, 0, 0); +}
Userspace specifies CLONE_VM to share address space and spawn new thread. `clone` allow userspace to specify a new stack for new thread. However there is no way to specify new shadow stack base address without changing API. This patch allocates a new shadow stack whenever CLONE_VM is given. In case of CLONE_VFORK, parent is suspended until child finishes and thus can child use parent shadow stack. In case of !CLONE_VM, COW kicks in because entire address space is copied from parent to child. `clone3` is extensible and can provide mechanisms using which shadow stack as an input parameter can be provided. This is not settled yet and being extensively discussed on mailing list. Once that's settled, this commit will adapt to that. Signed-off-by: Deepak Gupta <debug@rivosinc.com> --- arch/riscv/include/asm/usercfi.h | 25 ++++++++ arch/riscv/kernel/process.c | 11 +++- arch/riscv/kernel/usercfi.c | 121 +++++++++++++++++++++++++++++++++++++++ 3 files changed, 156 insertions(+), 1 deletion(-)