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40c45904f8
Debuggers expect that doing PTRACE_GETREGS, then poking at a tracee
and maybe letting it run for a while, then doing PTRACE_SETREGS will
put the tracee back where it was. In the specific case of a 32-bit
tracer and tracee, the PTRACE_GETREGS/SETREGS data structure doesn't
have fs_base or gs_base fields, so FSBASE and GSBASE fields are
never stored anywhere. Everything used to still work because
nonzero FS or GS would result full reloads of the segment registers
when the tracee resumes, and the bases associated with FS==0 or
GS==0 are irrelevant to 32-bit code.
Adding FSGSBASE support broke this: when FSGSBASE is enabled, FSBASE
and GSBASE are now restored independently of FS and GS for all tasks
when context-switched in. This means that, if a 32-bit tracer
restores a previous state using PTRACE_SETREGS but the tracee's
pre-restore and post-restore bases don't match, then the tracee is
resumed with the wrong base.
Fix it by explicitly loading the base when a 32-bit tracer pokes FS
or GS on a 64-bit kernel.
Also add a test case.
Fixes: 673903495c ("x86/process/64: Use FSBSBASE in switch_to() if available")
Signed-off-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/229cc6a50ecbb701abd50fe4ddaf0eda888898cd.1593192140.git.luto@kernel.org
86 lines
2.0 KiB
C
86 lines
2.0 KiB
C
/* SPDX-License-Identifier: GPL-2.0 */
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#ifndef _ASM_FSGSBASE_H
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#define _ASM_FSGSBASE_H
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#ifndef __ASSEMBLY__
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#ifdef CONFIG_X86_64
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#include <asm/msr-index.h>
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/*
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* Read/write a task's FSBASE or GSBASE. This returns the value that
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* the FS/GS base would have (if the task were to be resumed). These
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* work on the current task or on a non-running (typically stopped
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* ptrace child) task.
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*/
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extern unsigned long x86_fsbase_read_task(struct task_struct *task);
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extern unsigned long x86_gsbase_read_task(struct task_struct *task);
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extern void x86_fsbase_write_task(struct task_struct *task, unsigned long fsbase);
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extern void x86_gsbase_write_task(struct task_struct *task, unsigned long gsbase);
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/* Must be protected by X86_FEATURE_FSGSBASE check. */
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static __always_inline unsigned long rdfsbase(void)
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{
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unsigned long fsbase;
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asm volatile("rdfsbase %0" : "=r" (fsbase) :: "memory");
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return fsbase;
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}
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static __always_inline unsigned long rdgsbase(void)
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{
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unsigned long gsbase;
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asm volatile("rdgsbase %0" : "=r" (gsbase) :: "memory");
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return gsbase;
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}
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static __always_inline void wrfsbase(unsigned long fsbase)
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{
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asm volatile("wrfsbase %0" :: "r" (fsbase) : "memory");
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}
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static __always_inline void wrgsbase(unsigned long gsbase)
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{
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asm volatile("wrgsbase %0" :: "r" (gsbase) : "memory");
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}
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#include <asm/cpufeature.h>
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/* Helper functions for reading/writing FS/GS base */
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static inline unsigned long x86_fsbase_read_cpu(void)
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{
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unsigned long fsbase;
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if (static_cpu_has(X86_FEATURE_FSGSBASE))
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fsbase = rdfsbase();
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else
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rdmsrl(MSR_FS_BASE, fsbase);
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return fsbase;
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}
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static inline void x86_fsbase_write_cpu(unsigned long fsbase)
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{
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if (static_cpu_has(X86_FEATURE_FSGSBASE))
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wrfsbase(fsbase);
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else
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wrmsrl(MSR_FS_BASE, fsbase);
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}
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extern unsigned long x86_gsbase_read_cpu_inactive(void);
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extern void x86_gsbase_write_cpu_inactive(unsigned long gsbase);
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extern unsigned long x86_fsgsbase_read_task(struct task_struct *task,
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unsigned short selector);
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#endif /* CONFIG_X86_64 */
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#endif /* __ASSEMBLY__ */
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#endif /* _ASM_FSGSBASE_H */
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