[PATCH] x86_64: Plug GS leak in arch_prctl()
In linux-2.6.16, we have noticed a problem where the gs base value returned from an arch_prtcl(ARCH_GET_GS, ...) call will be incorrect if: - the current/calling task has NOT set its own gs base yet to a non-zero value, - some other task that ran on the same processor previously set their own gs base to a non-zero value. In this situation, the ARCH_GET_GS code will read and return the MSR_KERNEL_GS_BASE msr register. However, since the __switch_to() code does NOT load/zero the MSR_KERNEL_GS_BASE register when the task that is switched IN has a zero next->gs value, the caller of arch_prctl(ARCH_GET_GS, ...) will get back the value of some previous tasks's gs base value instead of 0. Change the arch_prctl() ARCH_GET_GS code to only read and return the MSR_KERNEL_GS_BASE msr register if the 'gs' register of the calling task is non-zero. Side note: Since in addition to using arch_prctl(ARCH_SET_GS, ...), a task can also setup a gs base value by using modify_ldt() and write an index value into 'gs' from user space, the patch below reads 'gs' instead of using thread.gs, since in the modify_ldt() case, the thread.gs value will be 0, and incorrect value would be returned (the task->thread.gs value). When the user has not set its own gs base value and the 'gs' register is zero, then the MSR_KERNEL_GS_BASE register will not be read and a value of zero will be returned by reading and returning 'task->thread.gs'. The first patch shown below is an attempt at implementing this approach. Signed-off-by: Andi Kleen <ak@suse.de> Signed-off-by: Linus Torvalds <torvalds@osdl.org>
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@ -781,10 +781,16 @@ long do_arch_prctl(struct task_struct *task, int code, unsigned long addr)
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}
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case ARCH_GET_GS: {
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unsigned long base;
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unsigned gsindex;
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if (task->thread.gsindex == GS_TLS_SEL)
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base = read_32bit_tls(task, GS_TLS);
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else if (doit)
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rdmsrl(MSR_KERNEL_GS_BASE, base);
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else if (doit) {
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asm("movl %%gs,%0" : "=r" (gsindex));
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if (gsindex)
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rdmsrl(MSR_KERNEL_GS_BASE, base);
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else
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base = task->thread.gs;
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}
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else
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base = task->thread.gs;
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ret = put_user(base, (unsigned long __user *)addr);
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