[tip: x86/fsgsbase] x86/ptrace: Prevent ptrace from clearing the FS/GS selector

[tip-bot2 for Chang S. Bae]

The following commit has been merged into the x86/fsgsbase branch of tip:

Commit-ID:     fddf8ba1e48860211c9639d00883833b42fcc1e0
Gitweb:        https://git.kernel.org/tip/fddf8ba1e48860211c9639d00883833b42fcc1e0
Author:        Chang S. Bae <chang.seok.bae@xxxxxxxxx>
AuthorDate:    Thu, 28 May 2020 16:13:47 -04:00
Committer:     Thomas Gleixner <tglx@xxxxxxxxxxxxx>
CommitterDate: Thu, 18 Jun 2020 15:46:59 +02:00

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@xxxxxxxxxx>
Signed-off-by: Chang S. Bae <chang.seok.bae@xxxxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Link: https://lkml.kernel.org/r/9040CFCD-74BD-4C17-9A01-B9B713CF6B10@xxxxxxxxx
Link: https://lkml.kernel.org/r/20200528201402.1708239-2-sashal@xxxxxxxxxx


---
 arch/x86/kernel/ptrace.c | 17 ++---------------
 1 file changed, 2 insertions(+), 15 deletions(-)

diff --git a/arch/x86/kernel/ptrace.c b/arch/x86/kernel/ptrace.c
index 4413058..1c7646c 100644
--- a/arch/x86/kernel/ptrace.c
+++ b/arch/x86/kernel/ptrace.c
@@ -379,25 +379,12 @@ 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.
-		 *
-		 * NB: This behavior is nonsensical and likely needs to
-		 * change when FSGSBASE support is added.
-		 */
-		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
 	}