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Message-Id: <2b5bb0f417a95dfdcb50c6c41702bb0455fc535a.1430091970.git.luto@kernel.org>
Date: Sun, 26 Apr 2015 16:47:59 -0700
From: Andy Lutomirski <luto@...nel.org>
To: x86@...nel.org
Cc: "H. Peter Anvin" <hpa@...or.com>,
Andy Lutomirski <luto@...capital.net>,
Borislav Petkov <bp@...en8.de>,
Denys Vlasenko <vda.linux@...glemail.com>,
Linus Torvalds <torvalds@...ux-foundation.org>,
Brian Gerst <brgerst@...il.com>,
Denys Vlasenko <dvlasenk@...hat.com>,
Ingo Molnar <mingo@...nel.org>,
Steven Rostedt <rostedt@...dmis.org>,
Oleg Nesterov <oleg@...hat.com>,
Frederic Weisbecker <fweisbec@...il.com>,
Alexei Starovoitov <ast@...mgrid.com>,
Will Drewry <wad@...omium.org>,
Kees Cook <keescook@...omium.org>,
Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
Andy Lutomirski <luto@...nel.org>
Subject: [PATCH v2] x86_64, asm: Work around AMD SYSRET SS descriptor attribute issue
AMD CPUs don't reinitialize the SS descriptor on SYSRET, so SYSRET
with SS == 0 results in an invalid usermode state in which SS is
apparently equal to __USER_DS but causes #SS if used.
Work around the issue by setting SS to __KERNEL_DS __switch_to, thus
ensuring that SYSRET never happens with SS set to NULL.
This was exposed by a recent vDSO cleanup.
Fixes: e7d6eefaaa44 x86/vdso32/syscall.S: Do not load __USER32_DS to %ss
Signed-off-by: Andy Lutomirski <luto@...nel.org>
---
Changes since v1:
- Improve comments per Denys' advice
- Force SS to __KERNEL_DS -- don't just check for NULL.
Please apply my test case patch, too.
arch/x86/ia32/ia32entry.S | 7 +++++++
arch/x86/include/asm/cpufeature.h | 1 +
arch/x86/kernel/cpu/amd.c | 3 +++
arch/x86/kernel/entry_64.S | 9 +++++++++
arch/x86/kernel/process_64.c | 28 ++++++++++++++++++++++++++++
5 files changed, 48 insertions(+)
diff --git a/arch/x86/ia32/ia32entry.S b/arch/x86/ia32/ia32entry.S
index 3c9fadf8b95c..c84b90bd30a7 100644
--- a/arch/x86/ia32/ia32entry.S
+++ b/arch/x86/ia32/ia32entry.S
@@ -421,6 +421,13 @@ sysretl_from_sys_call:
* cs and ss are loaded from MSRs.
* (Note: 32bit->32bit SYSRET is different: since r11
* does not exist, it merely sets eflags.IF=1).
+ *
+ * NB: On AMD CPUs with the X86_BUG_SYSRET_SS_ATTRS bug, the ss
+ * descriptor is not reinitialized. This means that we must
+ * avoid SYSRET with SS == NULL, which could happen if we schedule,
+ * exit the kernel, and re-enter using an interrupt vector. (All
+ * interrupt entries on x86_64 set SS to NULL.) We prevent that
+ * from happening by reloading SS in __switch_to.
*/
USERGS_SYSRET32
diff --git a/arch/x86/include/asm/cpufeature.h b/arch/x86/include/asm/cpufeature.h
index 854c04b3c9c2..7e244f626301 100644
--- a/arch/x86/include/asm/cpufeature.h
+++ b/arch/x86/include/asm/cpufeature.h
@@ -257,6 +257,7 @@
#define X86_BUG_11AP X86_BUG(5) /* Bad local APIC aka 11AP */
#define X86_BUG_FXSAVE_LEAK X86_BUG(6) /* FXSAVE leaks FOP/FIP/FOP */
#define X86_BUG_CLFLUSH_MONITOR X86_BUG(7) /* AAI65, CLFLUSH required before MONITOR */
+#define X86_BUG_SYSRET_SS_ATTRS X86_BUG(8) /* SYSRET doesn't fix up SS attrs */
#if defined(__KERNEL__) && !defined(__ASSEMBLY__)
diff --git a/arch/x86/kernel/cpu/amd.c b/arch/x86/kernel/cpu/amd.c
index fd470ebf924e..e4cf63301ff4 100644
--- a/arch/x86/kernel/cpu/amd.c
+++ b/arch/x86/kernel/cpu/amd.c
@@ -720,6 +720,9 @@ static void init_amd(struct cpuinfo_x86 *c)
if (!cpu_has(c, X86_FEATURE_3DNOWPREFETCH))
if (cpu_has(c, X86_FEATURE_3DNOW) || cpu_has(c, X86_FEATURE_LM))
set_cpu_cap(c, X86_FEATURE_3DNOWPREFETCH);
+
+ /* AMD CPUs don't reset SS attributes on SYSRET */
+ set_cpu_bug(c, X86_BUG_SYSRET_SS_ATTRS);
}
#ifdef CONFIG_X86_32
diff --git a/arch/x86/kernel/entry_64.S b/arch/x86/kernel/entry_64.S
index 0034012d04ea..2a14b1568f8a 100644
--- a/arch/x86/kernel/entry_64.S
+++ b/arch/x86/kernel/entry_64.S
@@ -295,6 +295,15 @@ system_call_fastpath:
* rflags from r11 (but RF and VM bits are forced to 0),
* cs and ss are loaded from MSRs.
* Restoration of rflags re-enables interrupts.
+ *
+ * NB: On AMD CPUs with the X86_BUG_SYSRET_SS_ATTRS bug, the ss
+ * descriptor is not reinitialized. This means that we should
+ * avoid SYSRET with SS == NULL, which could happen if we schedule,
+ * exit the kernel, and re-enter using an interrupt vector. (All
+ * interrupt entries on x86_64 set SS to NULL.) We prevent that
+ * from happening by reloading SS in __switch_to. (Actually
+ * detecting the failure in 64-bit userspace is tricky but can be
+ * done.)
*/
USERGS_SYSRET64
diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c
index 4baaa972f52a..ddfdbf74f174 100644
--- a/arch/x86/kernel/process_64.c
+++ b/arch/x86/kernel/process_64.c
@@ -419,6 +419,34 @@ __switch_to(struct task_struct *prev_p, struct task_struct *next_p)
task_thread_info(prev_p)->flags & _TIF_WORK_CTXSW_PREV))
__switch_to_xtra(prev_p, next_p, tss);
+ if (static_cpu_has_bug(X86_BUG_SYSRET_SS_ATTRS)) {
+ /*
+ * AMD CPUs have a misfeature: SYSRET sets the SS selector but
+ * does not update the cached descriptor. As a result, if we
+ * do SYSRET while SS is NULL, we'll end up in user mode with
+ * SS apparently equal to __USER_DS but actually unusable.
+ *
+ * The straightforward workaround would be to fix it up just
+ * before SYSRET, but that would slow down the system call
+ * fast paths. Instead, we ensure that SS is never NULL in
+ * system call context. We do this by replacing NULL SS
+ * selectors at every context switch. SYSCALL sets up a valid
+ * SS, so the only way to get NULL is to re-enter the kernel
+ * from CPL 3 through an interrupt. Since that can't happen
+ * in the same task as a running syscall, we are guaranteed to
+ * context switch between every interrupt vector entry and a
+ * subsequent SYSRET.
+ *
+ * We read SS first because SS reads are much faster than
+ * writes. Out of caution, we force SS to __KERNEL_DS even if
+ * it previously had a different non-NULL value.
+ */
+ unsigned short ss_sel;
+ savesegment(ss, ss_sel);
+ if (ss_sel != __KERNEL_DS)
+ loadsegment(ss, __KERNEL_DS);
+ }
+
return prev_p;
}
--
2.3.0
--
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