[tip: x86/urgent] x86/dumpstack: Do not try to access user space code of other tasks

[tip-bot2 for Thomas Gleixner]

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

Commit-ID:     860aaabac8235cfde10fe556aa82abbbe3117888
Gitweb:        https://git.kernel.org/tip/860aaabac8235cfde10fe556aa82abbbe3117888
Author:        Thomas Gleixner <tglx@xxxxxxxxxxxxx>
AuthorDate:    Tue, 17 Nov 2020 21:23:34 +01:00
Committer:     Borislav Petkov <bp@xxxxxxx>
CommitterDate: Wed, 18 Nov 2020 12:56:29 +01:00

x86/dumpstack: Do not try to access user space code of other tasks

sysrq-t ends up invoking show_opcodes() for each task which tries to access
the user space code of other processes, which is obviously bogus.

It either manages to dump where the foreign task's regs->ip points to in a
valid mapping of the current task or triggers a pagefault and prints "Code:
Bad RIP value.". Both is just wrong.

Add a safeguard in copy_code() and check whether the @regs pointer matches
currents pt_regs. If not, do not even try to access it.

While at it, add commentary why using copy_from_user_nmi() is safe in
copy_code() even if the function name suggests otherwise.

Reported-by: Oleg Nesterov <oleg@xxxxxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Signed-off-by: Borislav Petkov <bp@xxxxxxx>
Reviewed-by: Borislav Petkov <bp@xxxxxxx>
Acked-by: Oleg Nesterov <oleg@xxxxxxxxxx>
Tested-by: Borislav Petkov <bp@xxxxxxx>
Link: https://lkml.kernel.org/r/20201117202753.667274723@xxxxxxxxxxxxx
---
 arch/x86/kernel/dumpstack.c | 23 +++++++++++++++++++----
 1 file changed, 19 insertions(+), 4 deletions(-)

diff --git a/arch/x86/kernel/dumpstack.c b/arch/x86/kernel/dumpstack.c
index 25c06b6..97aa900 100644
--- a/arch/x86/kernel/dumpstack.c
+++ b/arch/x86/kernel/dumpstack.c
@@ -78,6 +78,9 @@ static int copy_code(struct pt_regs *regs, u8 *buf, unsigned long src,
 	if (!user_mode(regs))
 		return copy_from_kernel_nofault(buf, (u8 *)src, nbytes);
 
+	/* The user space code from other tasks cannot be accessed. */
+	if (regs != task_pt_regs(current))
+		return -EPERM;
 	/*
 	 * Make sure userspace isn't trying to trick us into dumping kernel
 	 * memory by pointing the userspace instruction pointer at it.
@@ -85,6 +88,12 @@ static int copy_code(struct pt_regs *regs, u8 *buf, unsigned long src,
 	if (__chk_range_not_ok(src, nbytes, TASK_SIZE_MAX))
 		return -EINVAL;
 
+	/*
+	 * Even if named copy_from_user_nmi() this can be invoked from
+	 * other contexts and will not try to resolve a pagefault, which is
+	 * the correct thing to do here as this code can be called from any
+	 * context.
+	 */
 	return copy_from_user_nmi(buf, (void __user *)src, nbytes);
 }
 
@@ -115,13 +124,19 @@ void show_opcodes(struct pt_regs *regs, const char *loglvl)
 	u8 opcodes[OPCODE_BUFSIZE];
 	unsigned long prologue = regs->ip - PROLOGUE_SIZE;
 
-	if (copy_code(regs, opcodes, prologue, sizeof(opcodes))) {
-		printk("%sCode: Unable to access opcode bytes at RIP 0x%lx.\n",
-		       loglvl, prologue);
-	} else {
+	switch (copy_code(regs, opcodes, prologue, sizeof(opcodes))) {
+	case 0:
 		printk("%sCode: %" __stringify(PROLOGUE_SIZE) "ph <%02x> %"
 		       __stringify(EPILOGUE_SIZE) "ph\n", loglvl, opcodes,
 		       opcodes[PROLOGUE_SIZE], opcodes + PROLOGUE_SIZE + 1);
+		break;
+	case -EPERM:
+		/* No access to the user space stack of other tasks. Ignore. */
+		break;
+	default:
+		printk("%sCode: Unable to access opcode bytes at RIP 0x%lx.\n",
+		       loglvl, prologue);
+		break;
 	}
 }