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charset="utf-8" Adding documentation on shadow stack for user mode on riscv and kernel interfaces exposed so that user tasks can enable it. Signed-off-by: Deepak Gupta --- Documentation/arch/riscv/zicfiss.rst | 169 +++++++++++++++++++++++++++ 1 file changed, 169 insertions(+) create mode 100644 Documentation/arch/riscv/zicfiss.rst diff --git a/Documentation/arch/riscv/zicfiss.rst b/Documentation/arch/risc= v/zicfiss.rst new file mode 100644 index 000000000000..f133b6af9c15 --- /dev/null +++ b/Documentation/arch/riscv/zicfiss.rst @@ -0,0 +1,169 @@ +.. SPDX-License-Identifier: GPL-2.0 + +:Author: Deepak Gupta +:Date: 12 January 2024 + +=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D=3D=3D=3D=3D=3D +Shadow stack to protect function returns on RISC-V Linux +=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D=3D=3D=3D=3D=3D + +This document briefly describes the interface provided to userspace by Lin= ux +to enable shadow stack for user mode applications on RISV-V + +1. Feature Overview +-------------------- + +Memory corruption issues usually result in to crashes, however when in han= ds of +an adversary and if used creatively can result into variety security issue= s. + +One of those security issues can be code re-use attacks on program where a= dversary +can use corrupt return addresses present on stack and chain them together = to perform +return oriented programming (ROP) and thus compromising control flow integ= rity (CFI) +of the program. + +Return addresses live on stack and thus in read-write memory and thus are +susceptible to corruption and allows an adversary to reach any program cou= nter +(PC) in address space. On RISC-V `zicfiss` extension provides an alternate= stack +`shadow stack` on which return addresses can be safely placed in prolog of= the +function and retrieved in epilog. `zicfiss` extension makes following chan= ges + + - PTE encodings for shadow stack virtual memory + An earlier reserved encoding in first stage translation i.e. + PTE.R=3D0, PTE.W=3D1, PTE.X=3D0 becomes PTE encoding for shadow stack = pages. + + - `sspush x1/x5` instruction pushes (stores) `x1/x5` to shadow stack. + + - `sspopchk x1/x5` instruction pops (loads) from shadow stack and compares + with `x1/x5` and if un-equal, CPU raises `software check exception` with + `*tval =3D 3` + +Compiler toolchain makes sure that function prologs have `sspush x1/x5` to= save return +address on shadow stack in addition to regular stack. Similarly function e= pilogs have +`ld x5, offset(x2)`; `sspopchk x5` to ensure that popped value from regula= r stack +matches with popped value from shadow stack. + +2. Shadow stack protections and linux memory manager +----------------------------------------------------- + +As mentioned earlier, shadow stack get new page table encodings and thus h= ave some +special properties assigned to them and instructions that operate on them = as below + + - Regular stores to shadow stack memory raises access store faults. + This way shadow stack memory is protected from stray inadvertant + writes + + - Regular loads to shadow stack memory are allowed. + This allows stack trace utilities or backtrace functions to read + true callstack (not tampered) + + - Only shadow stack instructions can generate shadow stack load or + shadow stack store. + + - Shadow stack load / shadow stack store on read-only memory raises + AMO/store page fault. Thus both `sspush x1/x5` and `sspopchk x1/x5` + will raise AMO/store page fault. This simplies COW handling in kernel + During fork, kernel can convert shadow stack pages into read-only + memory (as it does for regular read-write memory) and as soon as + subsequent `sspush` or `sspopchk` in userspace is encountered, then + kernel can perform COW. + + - Shadow stack load / shadow stack store on read-write, read-write- + execute memory raises an access fault. This is a fatal condition + because shadow stack should never be operating on read-write, read- + write-execute memory. + +3. ELF and psABI +----------------- + +Toolchain sets up `GNU_PROPERTY_RISCV_FEATURE_1_BCFI` for property +`GNU_PROPERTY_RISCV_FEATURE_1_AND` in notes section of the object file. + +4. Linux enabling +------------------ + +User space programs can have multiple shared objects loaded in its address= space +and it's a difficult task to make sure all the dependencies have been comp= iled +with support of shadow stack. Thus it's left to dynamic loader to enable +shadow stack for the program. + +5. prctl() enabling +-------------------- + +`PR_SET_SHADOW_STACK_STATUS` / `PR_GET_SHADOW_STACK_STATUS` / +`PR_LOCK_SHADOW_STACK_STATUS` are three prctls added to manage shadow stack +enabling for tasks. prctls are arch agnostic and returns -EINVAL on other = arches. + +`PR_SET_SHADOW_STACK_STATUS`: If arg1 `PR_SHADOW_STACK_ENABLE` and if CPU = supports +`zicfiss` then kernel will enable shadow stack for the task. Dynamic loade= r can +issue this `prctl` once it has determined that all the objects loaded in a= ddress +space have support for shadow stack. Additionally if there is a `dlopen` t= o an +object which wasn't compiled with `zicfiss`, dynamic loader can issue this= prctl +with arg1 set to 0 (i.e. `PR_SHADOW_STACK_ENABLE` being clear) + +`PR_GET_SHADOW_STACK_STATUS`: Returns current status of indirect branch tr= acking. +If enabled it'll return `PR_SHADOW_STACK_ENABLE` + +`PR_LOCK_SHADOW_STACK_STATUS`: Locks current status of shadow stack enabli= ng on the +task. User space may want to run with strict security posture and wouldn't= want +loading of objects without `zicfiss` support in it and thus would want to = disallow +disabling of shadow stack on current task. In that case user space can use= this prctl +to lock current settings. + +5. violations related to returns with shadow stack enabled +----------------------------------------------------------- + +Pertaining to shadow stack, CPU raises software check exception in followi= ng +condition + + - On execution of `sspopchk x1/x5`, x1/x5 didn't match top of shadow stac= k. + If mismatch happens then cpu does `*tval =3D 3` and raise software check + exception + +Linux kernel will treat this as `SIGSEV`` with code =3D `SEGV_CPERR` and f= ollow +normal course of signal delivery. + +6. Shadow stack tokens +----------------------- +Regular stores on shadow stacks are not allowed and thus can't be tampered= with via +arbitrary stray writes due to bugs. Method of pivoting / switching to shad= ow stack +is simply writing to csr `CSR_SSP` changes active shadow stack. This can b= e problematic +because usually value to be written to `CSR_SSP` will be loaded somewhere = in writeable +memory and thus allows an adversary to corruption bug in software to pivot= to an any +address in shadow stack range. Shadow stack tokens can help mitigate this = problem by +making sure that: + + - When software is switching away from a shadow stack, shadow stack point= er should be + saved on shadow stack itself and call it `shadow stack token` + + - When software is switching to a shadow stack, it should read the `shado= w stack token` + from shadow stack pointer and verify that `shadow stack token` itself i= s pointer to + shadow stack itself. + + - Once the token verification is done, software can perform the write to = `CSR_SSP` to + switch shadow stack. + +Here software can be user mode task runtime itself which is managing vario= us contexts +as part of single thread. Software can be kernel as well when kernel has t= o deliver a +signal to user task and must save shadow stack pointer. Kernel can perform= similar +procedure by saving a token on user shadow stack itself. This way whenever= sigreturn +happens, kernel can read the token and verify the token and then switch to= shadow stack. +Using this mechanism, kernel helps user task so that any corruption issue = in user task +is not exploited by adversary by arbitrarily using `sigreturn`. Adversary = will have to +make sure that there is a `shadow stack token` in addition to invoking `si= greturn` + +7. Signal shadow stack +----------------------- +Following structure has been added to sigcontext for RISC-V. `rsvd` field = has been kept +in case we need some extra information in future for landing pads / indire= ct branch +tracking. It has been kept today in order to allow backward compatibility = in future. + +struct __sc_riscv_cfi_state { + unsigned long ss_ptr; + unsigned long rsvd; +}; + +As part of signal delivery, shadow stack token is saved on current shadow = stack itself and +updated pointer is saved away in `ss_ptr` field in `__sc_riscv_cfi_state` = under `sigcontext` +Existing shadow stack allocation is used for signal delivery. During `sigr= eturn`, kernel will +obtain `ss_ptr` from `sigcontext` and verify the saved token on shadow sta= ck itself and switch +shadow stack. --=20 2.45.0