Messages in this thread | | | Subject | Re: [PATCH v18 24/25] x86/cet/shstk: Add arch_prctl functions for shadow stack | From | Dave Hansen <> | Date | Fri, 29 Jan 2021 09:07:01 -0800 |
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On 1/27/21 1:25 PM, Yu-cheng Yu wrote: > arch_prctl(ARCH_X86_CET_STATUS, u64 *args) > Get CET feature status. > > The parameter 'args' is a pointer to a user buffer. The kernel returns > the following information: > > *args = shadow stack/IBT status > *(args + 1) = shadow stack base address > *(args + 2) = shadow stack size
What's the deal for 32-bit binaries? The in-kernel code looks 64-bit only, but I don't see anything restricting the interface to 64-bit.
> +static int copy_status_to_user(struct cet_status *cet, u64 arg2)
This has static scope, but it's still awfully generically named. A cet_ prefix would be nice.
> +{ > + u64 buf[3] = {0, 0, 0}; > + > + if (cet->shstk_size) { > + buf[0] |= GNU_PROPERTY_X86_FEATURE_1_SHSTK; > + buf[1] = (u64)cet->shstk_base; > + buf[2] = (u64)cet->shstk_size;
What's the casting for?
> + } > + > + return copy_to_user((u64 __user *)arg2, buf, sizeof(buf)); > +} > + > +int prctl_cet(int option, u64 arg2) > +{ > + struct cet_status *cet; > + unsigned int features; > + > + /* > + * GLIBC's ENOTSUPP == EOPNOTSUPP == 95, and it does not recognize > + * the kernel's ENOTSUPP (524). So return EOPNOTSUPP here. > + */ > + if (!IS_ENABLED(CONFIG_X86_CET)) > + return -EOPNOTSUPP;
Let's ignore glibc for a moment. What error code *should* the kernel be returning here? errno(3) says:
EOPNOTSUPP Operation not supported on socket (POSIX.1) ... ENOTSUP Operation not supported (POSIX.1)
> + cet = ¤t->thread.cet; > + > + if (option == ARCH_X86_CET_STATUS) > + return copy_status_to_user(cet, arg2);
What's the point of doing copy_status_to_user() if the processor doesn't support CET? In other words, shouldn't this be below the CPU feature check?
Also, please cast arg2 *here*. It becomes a user pointer here, not at the copy_to_user().
> + if (!static_cpu_has(X86_FEATURE_CET)) > + return -EOPNOTSUPP;
So, you went to the trouble of adding a disabled-features.h entry for this. Why not just do:
if (cpu_feature_enabled(X86_FEATURE_CET)) ...
instead of the IS_ENABLED() check above? That should get rid of one of these if's.
> + switch (option) { > + case ARCH_X86_CET_DISABLE: > + if (cet->locked) > + return -EPERM; > + > + features = (unsigned int)arg2;
What's the purpose of this cast?
> + if (features & ~GNU_PROPERTY_X86_FEATURE_1_VALID) > + return -EINVAL; > + if (features & GNU_PROPERTY_X86_FEATURE_1_SHSTK) > + cet_disable_shstk(); > + return 0;
This doesn't enforce that the high bits of arg2 be 0. Shouldn't we call them reserved and enforce that they be 0?
> + case ARCH_X86_CET_LOCK: > + cet->locked = 1; > + return 0;
This needs to check for and enforce that arg2==0.
> + default: > + return -ENOSYS; > + } > +}
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