linux-kernel - [PATCH v5 4/5] x86_64,entry: Treat regs->ax the same in fastpath and slowpath syscalls

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-Id: <01218b493f12ae2f98034b78c9ae085e38e94350.1409954077.git.luto@amacapital.net>
Date:	Fri,  5 Sep 2014 15:13:55 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	linux-kernel@...r.kernel.org, Kees Cook <keescook@...omium.org>,
	Will Drewry <wad@...omium.org>, Oleg Nesterov <oleg@...hat.com>
Cc:	x86@...nel.org, linux-arm-kernel@...ts.infradead.org,
	linux-mips@...ux-mips.org, linux-arch@...r.kernel.org,
	linux-security-module@...r.kernel.org,
	Alexei Starovoitov <ast@...mgrid.com>, hpa@...or.com,
	Frederic Weisbecker <fweisbec@...il.com>,
	Andy Lutomirski <luto@...capital.net>
Subject: [PATCH v5 4/5] x86_64,entry: Treat regs->ax the same in fastpath and slowpath syscalls

For slowpath syscalls, we initialize regs->ax to -ENOSYS and stick
the syscall number into regs->orig_ax prior to any possible tracing
and syscall execution.  This is user-visible ABI used by ptrace
syscall emulation and seccomp.

For fastpath syscalls, there's no good reason not to do the same
thing.  It's even slightly simpler than what we're currently doing.
It probably has no measureable performance impact.  It should have
no user-visible effect.

The purpose of this patch is to prepare for two-phase syscall
tracing, in which the first phase might modify the saved RAX without
leaving the fast path.  This change is just subtle enough that I'm
keeping it separate.

Signed-off-by: Andy Lutomirski <luto@...capital.net>
---
 arch/x86/include/asm/calling.h |  6 +++++-
 arch/x86/kernel/entry_64.S     | 13 ++++---------
 2 files changed, 9 insertions(+), 10 deletions(-)

diff --git a/arch/x86/include/asm/calling.h b/arch/x86/include/asm/calling.h
index cb4c73bfeb48..76659b67fd11 100644
--- a/arch/x86/include/asm/calling.h
+++ b/arch/x86/include/asm/calling.h
@@ -85,7 +85,7 @@ For 32-bit we have the following conventions - kernel is built with
 #define ARGOFFSET	R11
 #define SWFRAME		ORIG_RAX
 
-	.macro SAVE_ARGS addskip=0, save_rcx=1, save_r891011=1
+	.macro SAVE_ARGS addskip=0, save_rcx=1, save_r891011=1, rax_enosys=0
 	subq  $9*8+\addskip, %rsp
 	CFI_ADJUST_CFA_OFFSET	9*8+\addskip
 	movq_cfi rdi, 8*8
@@ -96,7 +96,11 @@ For 32-bit we have the following conventions - kernel is built with
 	movq_cfi rcx, 5*8
 	.endif
 
+	.if \rax_enosys
+	movq $-ENOSYS, 4*8(%rsp)
+	.else
 	movq_cfi rax, 4*8
+	.endif
 
 	.if \save_r891011
 	movq_cfi r8,  3*8
diff --git a/arch/x86/kernel/entry_64.S b/arch/x86/kernel/entry_64.S
index 2fac1343a90b..0bd6d3c28064 100644
--- a/arch/x86/kernel/entry_64.S
+++ b/arch/x86/kernel/entry_64.S
@@ -404,8 +404,8 @@ GLOBAL(system_call_after_swapgs)
 	 * and short:
 	 */
 	ENABLE_INTERRUPTS(CLBR_NONE)
-	SAVE_ARGS 8,0
-	movq  %rax,ORIG_RAX-ARGOFFSET(%rsp)
+	SAVE_ARGS 8, 0, rax_enosys=1
+	movq_cfi rax,(ORIG_RAX-ARGOFFSET)
 	movq  %rcx,RIP-ARGOFFSET(%rsp)
 	CFI_REL_OFFSET rip,RIP-ARGOFFSET
 	testl $_TIF_WORK_SYSCALL_ENTRY,TI_flags+THREAD_INFO(%rsp,RIP-ARGOFFSET)
@@ -417,7 +417,7 @@ system_call_fastpath:
 	andl $__SYSCALL_MASK,%eax
 	cmpl $__NR_syscall_max,%eax
 #endif
-	ja badsys
+	ja ret_from_sys_call  /* and return regs->ax */
 	movq %r10,%rcx
 	call *sys_call_table(,%rax,8)  # XXX:	 rip relative
 	movq %rax,RAX-ARGOFFSET(%rsp)
@@ -476,10 +476,6 @@ sysret_signal:
 	FIXUP_TOP_OF_STACK %r11, -ARGOFFSET
 	jmp int_check_syscall_exit_work
 
-badsys:
-	movq $-ENOSYS,RAX-ARGOFFSET(%rsp)
-	jmp ret_from_sys_call
-
 #ifdef CONFIG_AUDITSYSCALL
 	/*
 	 * Fast path for syscall audit without full syscall trace.
@@ -519,7 +515,6 @@ tracesys:
 	jz auditsys
 #endif
 	SAVE_REST
-	movq $-ENOSYS,RAX(%rsp) /* ptrace can change this for a bad syscall */
 	FIXUP_TOP_OF_STACK %rdi
 	movq %rsp,%rdi
 	call syscall_trace_enter
@@ -536,7 +531,7 @@ tracesys:
 	andl $__SYSCALL_MASK,%eax
 	cmpl $__NR_syscall_max,%eax
 #endif
-	ja   int_ret_from_sys_call	/* RAX(%rsp) set to -ENOSYS above */
+	ja   int_ret_from_sys_call	/* RAX(%rsp) is already set */
 	movq %r10,%rcx	/* fixup for C */
 	call *sys_call_table(,%rax,8)
 	movq %rax,RAX-ARGOFFSET(%rsp)
-- 
1.9.3

--
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/