The comment is ancient, it dates to the time when only AMD's x86_64 implementation existed. AMD wasn't (and still isn't) supporting SYSENTER, so these writes were "just in case" back then.
This has changed: Intel's x86_64 appeared, and Indel does support SYSENTER in long mode. "Some future 64-bit CPU" is here already. The code may appear "buggy" for AMD as it stands, since MSR_IA32_SYSENTER_EIP is only 32-bit for AMD CPUs. Writing a kernel function's address to it would drop high bits. Subsequent use of this MSR for branch via SYSENTER seem to allow user to transition to CPL0 while executing his code. Scary, eh? Explain why that is not a bug: because SYSENTER insn would not work on AMD CPU. Signed-off-by: Denys Vlasenko <[email protected]> CC: Linus Torvalds <[email protected]> CC: Steven Rostedt <[email protected]> CC: Ingo Molnar <[email protected]> CC: Borislav Petkov <[email protected]> CC: "H. Peter Anvin" <[email protected]> CC: Andy Lutomirski <[email protected]> CC: Oleg Nesterov <[email protected]> CC: Frederic Weisbecker <[email protected]> CC: Alexei Starovoitov <[email protected]> CC: Will Drewry <[email protected]> CC: Kees Cook <[email protected]> CC: [email protected] CC: [email protected] --- arch/x86/kernel/cpu/common.c | 6 ++++-- 1 file changed, 4 insertions(+), 2 deletions(-) diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c index b5ad0fa..7c57b12 100644 --- a/arch/x86/kernel/cpu/common.c +++ b/arch/x86/kernel/cpu/common.c @@ -1212,8 +1212,10 @@ void syscall_init(void) #ifdef CONFIG_IA32_EMULATION wrmsrl(MSR_CSTAR, ia32_cstar_target); /* - * Always load these, in case some future 64-bit CPU supports - * SYSENTER from compat mode too: + * This only works for Intel CPUs. + * On AMD, these MSRs are 32-bit, CPU truncates MSR_IA32_SYSENTER_EIP. + * This does not cause SYSENTER to jump to wrong location because + * AMD doesn't allow SYSENTER in long mode (either 32- or 64-bit). */ wrmsrl_safe(MSR_IA32_SYSENTER_CS, (u64)__KERNEL_CS); wrmsrl_safe(MSR_IA32_SYSENTER_ESP, 0ULL); -- 1.8.1.4 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
