Commit 61f01dd9 authored by Andy Lutomirski's avatar Andy Lutomirski Committed by Linus Torvalds

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: e7d6eefa x86/vdso32/syscall.S: Do not load __USER32_DS to %ss
Signed-off-by: default avatarAndy Lutomirski <luto@kernel.org>
Cc: Peter Anvin <hpa@zytor.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Denys Vlasenko <vda.linux@googlemail.com>
Cc: Brian Gerst <brgerst@gmail.com>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 1190944f
......@@ -427,6 +427,13 @@ 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
......
......@@ -265,6 +265,7 @@
#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__)
......
......@@ -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
......
......@@ -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
......
......@@ -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;
}
......
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