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Message-Id: <1427373731-13056-1-git-send-email-dvlasenk@redhat.com> Date: Thu, 26 Mar 2015 13:42:11 +0100 From: Denys Vlasenko <dvlasenk@...hat.com> To: Andy Lutomirski <luto@...capital.net> Cc: Denys Vlasenko <dvlasenk@...hat.com>, Borislav Petkov <bp@...en8.de>, x86@...nel.org, linux-kernel@...r.kernel.org Subject: [PATCH] x86/asm/entry/64: better check for canonical address This change makes the check exact (no more false positives on kernel addresses). It isn't really important to be fully correct here - almost all addresses we'll ever see will be userspace ones, but OTOH it looks to be cheap enough: the new code uses two more ALU ops but preserves %rcx, allowing to not reload it from pt_regs->cx again. On disassembly level, the changes are: cmp %rcx,0x80(%rsp) -> mov 0x80(%rsp),%r11; cmp %rcx,%r11 shr $0x2f,%rcx -> shl $0x10,%rcx; sar $0x10,%rcx; cmp %rcx,%r11 mov 0x58(%rsp),%rcx -> (eliminated) Signed-off-by: Denys Vlasenko <dvlasenk@...hat.com> CC: Borislav Petkov <bp@...en8.de> CC: x86@...nel.org CC: linux-kernel@...r.kernel.org --- Andy, I'd undecided myself on the merits of doing this. If you like it, feel free to take it in your tree. I trimmed CC list to not bother too many people with this trivial and quite possibly "useless churn"-class change. arch/x86/kernel/entry_64.S | 23 ++++++++++++----------- 1 file changed, 12 insertions(+), 11 deletions(-) diff --git a/arch/x86/kernel/entry_64.S b/arch/x86/kernel/entry_64.S index bf9afad..a36d04d 100644 --- a/arch/x86/kernel/entry_64.S +++ b/arch/x86/kernel/entry_64.S @@ -688,26 +688,27 @@ retint_swapgs: /* return to user-space */ * a completely clean 64-bit userspace context. */ movq RCX(%rsp),%rcx - cmpq %rcx,RIP(%rsp) /* RCX == RIP */ + movq RIP(%rsp),%r11 + cmpq %rcx,%r11 /* RCX == RIP */ jne opportunistic_sysret_failed /* * On Intel CPUs, sysret with non-canonical RCX/RIP will #GP * in kernel space. This essentially lets the user take over - * the kernel, since userspace controls RSP. It's not worth - * testing for canonicalness exactly -- this check detects any - * of the 17 high bits set, which is true for non-canonical - * or kernel addresses. (This will pessimize vsyscall=native. - * Big deal.) + * the kernel, since userspace controls RSP. * - * If virtual addresses ever become wider, this will need + * If width of "canonical tail" ever become variable, this will need * to be updated to remain correct on both old and new CPUs. */ .ifne __VIRTUAL_MASK_SHIFT - 47 .error "virtual address width changed -- sysret checks need update" .endif - shr $__VIRTUAL_MASK_SHIFT, %rcx - jnz opportunistic_sysret_failed + /* Change top 16 bits to be a sign-extension of the rest */ + shl $(64 - (__VIRTUAL_MASK_SHIFT+1)), %rcx + sar $(64 - (__VIRTUAL_MASK_SHIFT+1)), %rcx + /* If this changed %rcx, it was not canonical */ + cmpq %rcx, %r11 + jne opportunistic_sysret_failed cmpq $__USER_CS,CS(%rsp) /* CS must match SYSRET */ jne opportunistic_sysret_failed @@ -730,8 +731,8 @@ retint_swapgs: /* return to user-space */ */ irq_return_via_sysret: CFI_REMEMBER_STATE - /* r11 is already restored (see code above) */ - RESTORE_C_REGS_EXCEPT_R11 + /* rcx and r11 are already restored (see code above) */ + RESTORE_C_REGS_EXCEPT_RCX_R11 movq RSP(%rsp),%rsp USERGS_SYSRET64 CFI_RESTORE_STATE -- 1.8.1.4 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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