3.2.101-rc1 review patch. If anyone has any objections, please let me know. ------------------ From: Ben Hutchings <ben@xxxxxxxxxxxxxxx> commit 2fbd7af5af8665d18bcefae3e9700be07e22b681 upstream. The upstream version of this, touching C code, was written by Dan Williams, with the following description: > The syscall table base is a user controlled function pointer in kernel > space. Use array_index_nospec() to prevent any out of bounds speculation. > > While retpoline prevents speculating into a userspace directed target it > does not stop the pointer de-reference, the concern is leaking memory > relative to the syscall table base, by observing instruction cache > behavior. The x86_64 assembly version for 4.4 was written by Jiri Slaby, with the following description: > In 4.4.118, we have commit c8961332d6da (x86/syscall: Sanitize syscall > table de-references under speculation), which is a backport of upstream > commit 2fbd7af5af86. But it fixed only the C part of the upstream patch > -- the IA32 sysentry. So it ommitted completely the assembly part -- the > 64bit sysentry. > > Fix that in this patch by explicit array_index_mask_nospec written in > assembly. The same was used in lib/getuser.S. > > However, to have "sbb" working properly, we have to switch from "cmp" > against (NR_syscalls-1) to (NR_syscalls), otherwise the last syscall > number would be "and"ed by 0. It is because the original "ja" relies on > "CF" or "ZF", but we rely only on "CF" in "sbb". That means: switch to > "jae" conditional jump too. > > Final note: use rcx for mask as this is exactly what is overwritten by > the 4th syscall argument (r10) right after. In 3.2 the x86_32 syscall table lookup is also written in assembly. So I've taken Jiri's version and added similar masking in entry_32.S, using edx as the temporary. edx is clobbered by SAVE_REGS and seems to be free at this point. In 3.2 the x86_64 entry code also lacks syscall masking for x32. Cc: Dan Williams <dan.j.williams@xxxxxxxxx> Cc: Jiri Slaby <jslaby@xxxxxxx> Cc: Jan Beulich <JBeulich@xxxxxxxx> Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx> Cc: linux-arch@xxxxxxxxxxxxxxx Cc: kernel-hardening@xxxxxxxxxxxxxxxxxx Cc: gregkh@xxxxxxxxxxxxxxxxxxx Cc: Andy Lutomirski <luto@xxxxxxxxxx> Cc: alan@xxxxxxxxxxxxxxx Cc: Jinpu Wang <jinpu.wang@xxxxxxxxxxxxxxxx> Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx> --- --- a/arch/x86/kernel/entry_32.S +++ b/arch/x86/kernel/entry_32.S @@ -429,6 +429,8 @@ sysenter_past_esp: sysenter_do_call: cmpl $(nr_syscalls), %eax jae sysenter_badsys + sbb %edx, %edx /* array_index_mask_nospec() */ + and %edx, %eax call *sys_call_table(,%eax,4) sysenter_after_call: movl %eax,PT_EAX(%esp) @@ -512,6 +514,8 @@ ENTRY(system_call) cmpl $(nr_syscalls), %eax jae syscall_badsys syscall_call: + sbb %edx, %edx /* array_index_mask_nospec() */ + and %edx, %eax call *sys_call_table(,%eax,4) syscall_after_call: movl %eax,PT_EAX(%esp) # store the return value --- a/arch/x86/kernel/entry_64.S +++ b/arch/x86/kernel/entry_64.S @@ -517,8 +517,10 @@ ENTRY(system_call_after_swapgs) testl $_TIF_WORK_SYSCALL_ENTRY,TI_flags(%rcx) jnz tracesys system_call_fastpath: - cmpq $__NR_syscall_max,%rax - ja badsys + cmpq $NR_syscalls, %rax + jae badsys + sbb %rcx, %rcx /* array_index_mask_nospec() */ + and %rcx, %rax movq %r10,%rcx #ifdef CONFIG_RETPOLINE movq sys_call_table(, %rax, 8), %rax @@ -646,8 +648,10 @@ tracesys: */ LOAD_ARGS ARGOFFSET, 1 RESTORE_REST - cmpq $__NR_syscall_max,%rax - ja int_ret_from_sys_call /* RAX(%rsp) set to -ENOSYS above */ + cmpq $NR_syscalls, %rax + jae int_ret_from_sys_call /* RAX(%rsp) set to -ENOSYS above */ + sbb %rcx, %rcx /* array_index_mask_nospec() */ + and %rcx, %rax movq %r10,%rcx /* fixup for C */ #ifdef CONFIG_RETPOLINE movq sys_call_table(, %rax, 8), %rax