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Several recent exploits have used direct calls to the native_write_cr4() function to disable SMEP and SMAP before then continuing their exploits using userspace memory access. Direct calls of this form can be mitigate by pinning bits of CR4 so that they cannot be changed through a common function. This is not intended to be a general ROP protection (which would require CFI to defend against properly), but rather a way to avoid trivial direct function calling (or CFI bypasses via a matching function prototype) as seen in: https://googleprojectzero.blogspot.com/2017/05/exploiting-linux-kernel-via-packet.html (https://github.com/xairy/kernel-exploits/tree/master/CVE-2017-7308) The goals of this change: - Pin specific bits (SMEP, SMAP, and UMIP) when writing CR4. - Avoid setting the bits too early (they must become pinned only after CPU feature detection and selection has finished). - Pinning mask needs to be read-only during normal runtime. - Pinning needs to be checked after write to validate the cr4 state Using __ro_after_init on the mask is done so it can't be first disabled with a malicious write. Since these bits are global state (once established by the boot CPU and kernel boot parameters), they are safe to write to secondary CPUs before those CPUs have finished feature detection. As such, the bits are set at the first cr4 write, so that cr4 write bugs can be detected (instead of silently papered over). This uses a few bytes less storage of a location we don't have: read-only per-CPU data. A check is performed after the register write because an attack could just skip directly to the register write. Such a direct jump is possible because of how this function may be built by the compiler (especially due to the removal of frame pointers) where it doesn't add a stack frame (function exit may only be a retq without pops) which is sufficient for trivial exploitation like in the timer overwrites mentioned above). The asm argument constraints gain the "+" modifier to convince the compiler that it shouldn't make ordering assumptions about the arguments or memory, and treat them as changed. Signed-off-by: Kees Cook <keescook@chromium.org> Signed-off-by: Thomas Gleixner <tglx@linutronix.de> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Dave Hansen <dave.hansen@intel.com> Cc: kernel-hardening@lists.openwall.com Link: https://lkml.kernel.org/r/20190618045503.39105-3-keescook@chromium.org |
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