random code execution - kernel oops

unsigned long hx = 0x4bfcc50c;
int main()
{
asm("bl hx");
}

yields:

[101274.818295] Unable to handle kernel paging request for data at address 0x0ffdc000
[101274.818313] Faulting instruction address: 0xc00122a8
[101274.818330] Oops: Kernel access of bad area, sig: 11 [#11]
[101274.818335] PREEMPT PowerMac
[101274.818341] Modules linked in: nls_iso8859_15 isofs zlib_inflate udf af_packet binfmt_misc radeon drm hci_usb rfcomm l2cap bluetooth snd_powermac configfs nls_utf8 hfsplus nls_base fuse dm_snapshot dm_mirror sha256 joydev snd_aoa_codec_tas snd_aoa_fabric_layout appletouch snd_aoa usbhid firewire_ohci firewire_core crc_itu_t bcm43xx ieee80211softmac ieee80211 ieee80211_crypt arc4 snd_aoa_i2sbus snd_pcm_oss snd_mixer_oss snd_pcm snd_timer snd_page_alloc rc80211_simple snd soundcore ohci1394 ieee1394 snd_aoa_soundbus bcm43xx_mac80211 ssb ehci_hcd pcmcia firmware_class mac80211 ohci_hcd cfg80211 yenta_socket rsrc_nonstatic usbcore uninorth_agp pcmcia_core agpgart evdev unix
[101274.818448] NIP: c00122a8 LR: c0015950 CTR: 00000080
[101274.818456] REGS: cb157cd0 TRAP: 0300   Not tainted  (2.6.22-rc4-g7d59453a-dirty)
[101274.818463] MSR: 00009032 <EE,ME,IR,DR>  CR: 33003353  XER: 80000000
[101274.818478] DAR: 0ffdc000, DSISR: 40000000
[101274.818485] TASK = cfc56670[19956] '0x4bfcc50c' THREAD: cb156000
[101274.818490] GPR00: cfc0dc40 cb157d80 cfc56670 0ffdc000 00000080 22723101 0ffdc000 40000000 
[101274.818508] GPR08: c084e000 cfc0dc40 00000000 c084e000 0000015b 100189a0 c0610000 0ffdc000 
[101274.818525] GPR16: c05f8a10 100d0000 fe3fffff 00000000 ca437200 00000000 d2b170fc c0610000 
[101274.818542] GPR24: cfc0dc40 00000f70 0ffdceac ec849a58 ec849a58 0ffdceac 22723101 c0c9c460 
[101274.818560] NIP [c00122a8] __flush_dcache_icache+0x14/0x40
[101274.818580] LR [c0015950] update_mmu_cache+0xec/0xf0
[101274.818591] Call Trace:
[101274.818596] [cb157d80] [00000f70] 0xf70 (unreliable)
[101274.818610] [cb157da0] [c0079cec] __handle_mm_fault+0x2d8/0xbe4
[101274.818623] [cb157e10] [c0301aa8] do_page_fault+0x41c/0x554
[101274.818640] [cb157f40] [c00119f4] handle_page_fault+0xc/0x80
[101274.818650] --- Exception: 401 at 0xffdceac
[101274.818660]     LR = 0x1000043c
[101274.818664] Instruction dump:
[101274.818670] 4d820020 7c8903a6 7c001bac 38630020 4200fff8 7c0004ac 4e800020 60000000 
[101274.818687] 54630026 38800080 7c8903a6 7c661b78 <7c00186c> 38630020 4200fff8 7c0004ac 
[101274.818707] note: 0x4bfcc50c[19956] exited with preempt_count 2
[101274.818716] BUG: sleeping function called from invalid context at kernel/rwsem.c:20
[101274.818723] in_atomic():1, irqs_disabled():0
[101274.818727] Call Trace:
[101274.818732] [cb157bc0] [c0008e10] show_stack+0x3c/0x194 (unreliable)
[101274.818748] [cb157bf0] [c0027648] __might_sleep+0xd0/0xec
[101274.818764] [cb157c00] [c00494d4] down_read+0x24/0x5c
[101274.818778] [cb157c20] [c005cda4] acct_collect+0x44/0x1a4
[101274.818793] [cb157c40] [c0030470] do_exit+0x10c/0x8c4
[101274.818805] [cb157c80] [c000ff34] die+0x210/0x218
[101274.818815] [cb157cb0] [c0015600] bad_page_fault+0x90/0xd8
[101274.818825] [cb157cc0] [c0011a64] handle_page_fault+0x7c/0x80
[101274.818835] --- Exception: 300 at __flush_dcache_icache+0x14/0x40
[101274.818846]     LR = update_mmu_cache+0xec/0xf0
[101274.818852] [cb157d80] [00000f70] 0xf70 (unreliable)
[101274.818901] [cb157da0] [c0079cec] __handle_mm_fault+0x2d8/0xbe4
[101274.818911] [cb157e10] [c0301aa8] do_page_fault+0x41c/0x554
[101274.818923] [cb157f40] [c00119f4] handle_page_fault+0xc/0x80
[101274.818933] --- Exception: 401 at 0xffdceac
[101274.818942]     LR = 0x1000043c
[101274.818961] BUG: scheduling while atomic: 0x4bfcc50c/0x10000002/19956
[101274.818967] Call Trace:
[101274.818971] [cb157ac0] [c0008e10] show_stack+0x3c/0x194 (unreliable)
[101274.818984] [cb157af0] [c02fe44c] schedule+0x584/0x6b4
[101274.818994] [cb157b40] [c00276f4] __cond_resched+0x34/0x60
[101274.819006] [cb157b50] [c02fe8f4] cond_resched+0x50/0x58
[101274.819016] [cb157b60] [c0077964] unmap_vmas+0x698/0x6b4
[101274.819026] [cb157be0] [c007c558] exit_mmap+0x74/0x120
[101274.819036] [cb157c10] [c002a1f0] mmput+0x68/0xf8
[101274.819048] [cb157c20] [c002e7fc] exit_mm+0xac/0x110
[101274.819058] [cb157c40] [c0030484] do_exit+0x120/0x8c4
[101274.819067] [cb157c80] [c000ff34] die+0x210/0x218
[101274.819077] [cb157cb0] [c0015600] bad_page_fault+0x90/0xd8
[101274.819087] [cb157cc0] [c0011a64] handle_page_fault+0x7c/0x80
[101274.819097] --- Exception: 300 at __flush_dcache_icache+0x14/0x40
[101274.819109]     LR = update_mmu_cache+0xec/0xf0
[101274.819115] [cb157d80] [00000f70] 0xf70 (unreliable)
[101274.819125] [cb157da0] [c0079cec] __handle_mm_fault+0x2d8/0xbe4
[101274.819135] [cb157e10] [c0301aa8] do_page_fault+0x41c/0x554
[101274.819147] [cb157f40] [c00119f4] handle_page_fault+0xc/0x80
[101274.819157] --- Exception: 401 at 0xffdceac
[101274.819166]     LR = 0x1000043c

The "is_exec" branch of the protection check in do_page_fault() didn't do anything on 32-bit PowerPC. So if a userland program jumps to a page with Linux protection flags "---p", all the tests happily fall through, and handle_mm_fault() is called, which in turn calls handle_pte_fault(), which calls update_mmu_cache(), which goes flush the dcache to a page with no access rights.

Boom.

---
Johannes, please test. Paul, I think you'll want this for .22 still? If the patch is correct, anyway ;-)

 arch/powerpc/mm/fault.c |    5 ++---
 1 files changed, 2 insertions(+), 3 deletions(-)

index bfe9013..115b25f 100644
@@ -279,14 +279,13 @@ good_area:
 #endif /* CONFIG_8xx */
 
 	if (is_exec) {
-#ifdef CONFIG_PPC64
+#if !(defined(CONFIG_4xx) || defined(CONFIG_BOOKE))
 		/* protection fault */
 		if (error_code & DSISR_PROTFAULT)
 			goto bad_area;
 		if (!(vma->vm_flags & VM_EXEC))
 			goto bad_area;
-#endif
-#if defined(CONFIG_4xx) || defined(CONFIG_BOOKE)
+#else
 		pte_t *ptep;
 		pmd_t *pmdp;
 

1.5.2.1.144.gabc40-dirty

The net result of which is trying to execute code in a region without access permissions.

Segher dug into the problem and suggested the patch below which does indeed fix the problem:

---
 arch/powerpc/mm/fault.c |    5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

@@ -279,14 +279,13 @@ good_area:
 #endif /* CONFIG_8xx */
 
 	if (is_exec) {
-#ifdef CONFIG_PPC64
+#if !(defined(CONFIG_4xx) || defined(CONFIG_BOOKE))
 		/* protection fault */
 		if (error_code & DSISR_PROTFAULT)
 			goto bad_area;
 		if (!(vma->vm_flags & VM_EXEC))
 			goto bad_area;
-#endif
-#if defined(CONFIG_4xx) || defined(CONFIG_BOOKE)
+#else
 		pte_t *ptep;
 		pmd_t *pmdp;