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authorBart Oldeman <bartoldeman@gmail.com>2010-09-23 13:16:58 -0400
committerGreg Kroah-Hartman <gregkh@suse.de>2010-11-22 11:03:05 -0800
commit8d080c8acf0ef106e5d1bf38590c47afde5a1bf2 (patch)
tree84946913815e9ccf3a1278e45872cd927ade180b /arch/x86
parentb6af39cd87b99525e0e6838bdee5819104cc1182 (diff)
x86, vm86: Fix preemption bug for int1 debug and int3 breakpoint handlers.
commit 6554287b1de0448f1e02e200d02b43914e997d15 upstream. Impact: fix kernel bug such as: BUG: scheduling while atomic: dosemu.bin/19680/0x00000004 See also Ubuntu bug 455067 at https://bugs.launchpad.net/ubuntu/+source/linux/+bug/455067 Commits 4915a35e35a037254550a2ba9f367a812bc37d40 ("Use preempt_conditional_sti/cli in do_int3, like on x86_64.") and 3d2a71a596bd9c761c8487a2178e95f8a61da083 ("x86, traps: converge do_debug handlers") started disabling preemption in int1 and int3 handlers on i386. The problem with vm86 is that the call to handle_vm86_trap() may jump straight to entry_32.S and never returns so preempt is never enabled again, and there is an imbalance in the preempt count. Commit be716615fe596ee117292dc615e95f707fb67fd1 ("x86, vm86: fix preemption bug"), which was later (accidentally?) reverted by commit 08d68323d1f0c34452e614263b212ca556dae47f ("hw-breakpoints: modifying generic debug exception to use thread-specific debug registers") fixed the problem for debug exceptions but not for breakpoints. There are three solutions to this problem. 1. Reenable preemption before calling handle_vm86_trap(). This was the approach that was later reverted. 2. Do not disable preemption for i386 in breakpoint and debug handlers. This was the situation before October 2008. As far as I understand preemption only needs to be disabled on x86_64 because a seperate stack is used, but it's nice to have things work the same way on i386 and x86_64. 3. Let handle_vm86_trap() return instead of jumping to assembly code. By setting a flag in _TIF_WORK_MASK, either TIF_IRET or TIF_NOTIFY_RESUME, the code in entry_32.S is instructed to return to 32 bit mode from V86 mode. The logic in entry_32.S was already present to handle signals. (I chose TIF_IRET because it's slightly more efficient in do_notify_resume() in signal.c, but in fact TIF_IRET can probably be replaced by TIF_NOTIFY_RESUME everywhere.) I'm submitting approach 3, because I believe it is the most elegant and prevents future confusion. Still, an obvious preempt_conditional_cli(regs); is necessary in traps.c to correct the bug. [ hpa: This is technically a regression, but because: 1. the regression is so old, 2. the patch seems relatively high risk, justifying more testing, and 3. we're late in the 2.6.36-rc cycle, I'm queuing it up for the 2.6.37 merge window. It might, however, justify as a -stable backport at a latter time, hence Cc: stable. ] Signed-off-by: Bart Oldeman <bartoldeman@users.sourceforge.net> LKML-Reference: <alpine.DEB.2.00.1009231312330.4732@localhost.localdomain> Cc: Frederic Weisbecker <fweisbec@gmail.com> Cc: K.Prasad <prasad@linux.vnet.ibm.com> Cc: Alan Stern <stern@rowland.harvard.edu> Cc: Alexander van Heukelum <heukelum@fastmail.fm> Signed-off-by: H. Peter Anvin <hpa@linux.intel.com> Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
Diffstat (limited to 'arch/x86')
-rw-r--r--arch/x86/kernel/traps.c1
-rw-r--r--arch/x86/kernel/vm86_32.c10
2 files changed, 9 insertions, 2 deletions
diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
index 60788dee0f8a..9f4edeb21323 100644
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -575,6 +575,7 @@ dotraplinkage void __kprobes do_debug(struct pt_regs *regs, long error_code)
if (regs->flags & X86_VM_MASK) {
handle_vm86_trap((struct kernel_vm86_regs *) regs,
error_code, 1);
+ preempt_conditional_cli(regs);
return;
}
diff --git a/arch/x86/kernel/vm86_32.c b/arch/x86/kernel/vm86_32.c
index 5ffb5622f793..61fb98519622 100644
--- a/arch/x86/kernel/vm86_32.c
+++ b/arch/x86/kernel/vm86_32.c
@@ -551,8 +551,14 @@ cannot_handle:
int handle_vm86_trap(struct kernel_vm86_regs *regs, long error_code, int trapno)
{
if (VMPI.is_vm86pus) {
- if ((trapno == 3) || (trapno == 1))
- return_to_32bit(regs, VM86_TRAP + (trapno << 8));
+ if ((trapno == 3) || (trapno == 1)) {
+ KVM86->regs32->ax = VM86_TRAP + (trapno << 8);
+ /* setting this flag forces the code in entry_32.S to
+ call save_v86_state() and change the stack pointer
+ to KVM86->regs32 */
+ set_thread_flag(TIF_IRET);
+ return 0;
+ }
do_int(regs, trapno, (unsigned char __user *) (regs->pt.ss << 4), SP(regs));
return 0;
}