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From: "Jan Beulich" <JBeulich@suse.com>
To: "xen-devel" <xen-devel@lists.xenproject.org>
Cc: Andrew Cooper <andrew.cooper3@citrix.com>, WeiLiu <wl@xen.org>,
	Roger Pau Monne <roger.pau@citrix.com>
Subject: [Xen-devel] [PATCH 1/2] x86/traps: guard top-of-stack reads
Date: Fri, 31 May 2019 03:17:53 -0600
Message-ID: <5CF0F1410200007800233D67@prv1-mh.provo.novell.com> (raw)
Message-ID: <20190531091753.hqPaHhWrTRK7d56GcYzWxI2Snle3a_0aQVWgJDvu5FU@z> (raw)
In-Reply-To: <5CF0ECE80200007800233D41@prv1-mh.provo.novell.com>

Nothing (afaics) guarantees that the original frame's stack pointer
points at readable memory. Avoid a (likely nested) crash by attaching
exception recovery to the read (making it a single read at the same
time). Don't even invoke _show_trace() in case of a non-readable top
slot.

Signed-off-by: Jan Beulich <jbeulich@suse.com>

--- a/xen/arch/x86/traps.c
+++ b/xen/arch/x86/traps.c
@@ -484,16 +484,23 @@ static void _show_trace(unsigned long sp
 
 static void show_trace(const struct cpu_user_regs *regs)
 {
-    unsigned long *sp = ESP_BEFORE_EXCEPTION(regs);
+    unsigned long *sp = ESP_BEFORE_EXCEPTION(regs), tos = 0;
 
     printk("Xen call trace:\n");
 
+    asm ( "1: mov %2,%0; 2:\n"
+          ".pushsection .fixup,\"ax\"\n"
+          "3: xor %k1,%k1; jmp 2b\n"
+          ".popsection\n"
+          _ASM_EXTABLE(1b, 3b)
+          : "+r" (tos), "+r" (sp) : "m" (*sp) );
+
     /*
      * If RIP looks sensible, or the top of the stack doesn't, print RIP at
      * the top of the stack trace.
      */
     if ( is_active_kernel_text(regs->rip) ||
-         !is_active_kernel_text(*sp) )
+         !is_active_kernel_text(tos) )
         printk("   [<%p>] %pS\n", _p(regs->rip), _p(regs->rip));
     /*
      * Else RIP looks bad but the top of the stack looks good.  Perhaps we
@@ -501,12 +508,15 @@ static void show_trace(const struct cpu_
      * return address; print it and skip past so _show_trace() doesn't print
      * it again.
      */
-    else
+    else if ( sp )
     {
-        printk("   [<%p>] %pS\n", _p(*sp), _p(*sp));
+        printk("   [<%p>] %pS\n", _p(tos), _p(tos));
         sp++;
     }
 
+    if ( !sp )
+        return;
+
     _show_trace((unsigned long)sp, regs->rbp);
 
     printk("\n");





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Thread overview: 19+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-05-31  8:59 [PATCH 0/2]: x86/traps: improve show_trace()'s top-of-stack handling Jan Beulich
2019-05-31  8:59 ` [Xen-devel] " Jan Beulich
2019-05-31  9:17 ` Jan Beulich [this message]
2019-05-31  9:17   ` [Xen-devel] [PATCH 1/2] x86/traps: guard top-of-stack reads Jan Beulich
2019-06-07 17:51   ` Andrew Cooper
2019-06-11  9:57     ` Jan Beulich
2019-05-31  9:22 ` [PATCH 2/2] x86/traps: widen condition for logging top-of-stack Jan Beulich
2019-05-31  9:22   ` [Xen-devel] " Jan Beulich
2019-06-07 18:01   ` Andrew Cooper
2019-06-11  9:46     ` Jan Beulich
2019-06-17  8:10 ` [Xen-devel] [PATCH v2 0/2]: x86/traps: improve show_trace()'s top-of-stack handling Jan Beulich
2019-06-17  8:12   ` [Xen-devel] [PATCH v2 1/2] x86/traps: guard top-of-stack reads Jan Beulich
2019-07-02 17:47     ` Andrew Cooper
2019-07-03  7:10       ` Jan Beulich
2019-06-17  8:13   ` [Xen-devel] [PATCH v2 2/2] x86/traps: widen condition for logging top-of-stack Jan Beulich
2019-07-03 10:21     ` Andrew Cooper
2019-07-03 10:34       ` Jan Beulich
2019-07-03 19:47         ` Andrew Cooper
2019-07-04  9:09           ` Jan Beulich

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