Commit 48f5e52e authored Jun 16, 2019 by Chang S. Bae Committed by Thomas Gleixner Jun 22, 2019

x86/ptrace: Prevent ptrace from clearing the FS/GS selector



When a ptracer writes a ptracee's FS/GSBASE with a different value, the
selector is also cleared. This behavior is not correct as the selector
should be preserved.

Update only the base value and leave the selector intact. To simplify the
code further remove the conditional checking for the same value as this
code is not performance critical.

The only recognizable downside of this change is when the selector is
already nonzero on write. The base will be reloaded according to the
selector. But the case is highly unexpected in real usages.

[ tglx: Massage changelog ]

Suggested-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Chang S. Bae <chang.seok.bae@intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: "H . Peter Anvin" <hpa@zytor.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Link: https://lkml.kernel.org/r/9040CFCD-74BD-4C17-9A01-B9B713CF6B10@intel.com

parent b302e4b1

arch/x86/kernel/ptrace.c

+2 −12

Original line number	Diff line number	Diff line
		@@ -397,22 +397,12 @@ static int putreg(struct task_struct *child,
		case offsetof(struct user_regs_struct,fs_base):
		if (value >= TASK_SIZE_MAX)
		return -EIO;
		/*
		* When changing the FS base, use do_arch_prctl_64()
		* to set the index to zero and to set the base
		* as requested.
		*/
		if (child->thread.fsbase != value)
		return do_arch_prctl_64(child, ARCH_SET_FS, value);
		x86_fsbase_write_task(child, value);
		return 0;
		case offsetof(struct user_regs_struct,gs_base):
		/*
		* Exactly the same here as the %fs handling above.
		*/
		if (value >= TASK_SIZE_MAX)
		return -EIO;
		if (child->thread.gsbase != value)
		return do_arch_prctl_64(child, ARCH_SET_GS, value);
		x86_gsbase_write_task(child, value);
		return 0;
		#endif
		}

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