Re: [Bisected] PowerMac G4 getting "BUG: Unable to handle kernel data access on write at 0x00001ff0" at boot with CONFIG_VMAP_STACK=y on kernels 6.5.x (regression over 6.4.x)

From: Liam R. Howlett
Date: Mon Sep 25 2023 - 21:49:56 EST

* Erhard Furtner <erhard_f@xxxxxxxxxxx> [230925 19:02]:
> Greetings!
>
> Had a chat on #gentoo-powerpc with another user whose G4 Mini fails booting kernel 6.5.0 when CONFIG_VMAP_STACK=y is enabled. I was able to replicate the issue on my PowerMac G4. Also I was able to bisect the issue.
>
> Kernels 6.4.x boot ok with CONFIG_VMAP_STACK=y but on 6.5.5 I get:
>
> [...]
> Kernel attempted to write user page (1ff0) - exploit attempt? (uid: 0)
> BUG: Unable to handle kernel data access on write at 0x00001ff0
> Faulting instruction address: 0xc0008750
> Oops: Kernel access of bad area, sig: 11 [#1]
> BE PAGE_SIZE=4K MMU=Hash PowerMac
> Modules linked in:
> CPU: 0 PID: 0 Comm: swapper Not tainted 6.5.5-PMacG4 #5
> Hardware name: PowerMac3,6 7455 0x80010303 PowerMac
> NIP: c0008750 LR: c0041848 CTR: c0070988
> REGS: c0d3dcd0 TRAP: 0300 Not tainted (6.5.5-PMacG4)
> MSR: 00001032 <ME,IR,DR,RI> CR: 22d3ddc0 XER: 20000000
> DAR: 00001ff0 DSISR: 42000000
> GPR00: c0041848 c0d3dd90 c0d06360 c0d3ddd0 c0d06360 c0d3dea8 c0d3adc0 00000000
> GPR08: 00000000 c0d40000 00000000 c0d3ddc0 00000000 00000000 00000000 00000004
> GPR16: 00000002 00000000 00000002 00402dc2 00402dc2 00002000 f1004000 00000000
> GPR24: c0d45220 c0d06644 c0843c34 00000002 c0d06360 c0d0ce00 c0d06360 00000000
> NIP [c0008750] do_softirq_own_stack+0x18/0x3c
> LR [c0041848] irq_exit+0x98/0xc4
> Call Trace:
> [c0d3dd90] [c0d69564] 0xc0d69564 (unreliable)
> [c0d3ddb0] [c0041848] irq_exit+0x98/0xc4
> [c0d3ddc0] [c0004a98] Decrementer_virt+0x108/0x10c
> --- interrupt: 900 at __schedule+0x43c/0x4e0
> NIP: c0843940 LR: c084398c CTR: c0070988
> REGS: c0d3ddd0 TRAP: 0900 Not tainted (6.5.5-PMacG4)
> MSR: 00009032 <EE,ME,IR,DR,RI> CR: 22024484 XER: 00000000
>
> GPR00: c0843574 c0d3de90 c0d06360 c0d06360 c0d06360 c0d3dea8 00000001 00000000
> GPR08: 00000000 00009032 c099ce2c 0007ffbf 22024484 00000000 00000000 00000004
> GPR16: 00000002 00000000 00000002 00402dc2 00402dc2 00002000 f1004000 00000000
> GPR24: c0d45220 c0d06644 c0843c34 00000002 c0d06360 c0d0ce00 c0d06360 c0d063ac
> NIP [c0843940] __schedule+0x43c/0x4e0
> LR [c084390c] __schedule+0x408/0x4e0
> --- interrupt: 900
> [c0d3de90] [c0843574] __schedule+0x70/0x4e0 (unreliable)
> [c0d3ded0] [c0843c34] __cond_resched+0x34/0x54
> [c0d3dee0] [c0141068] __vmalloc_node_range+0x27c/0x64c
> [c0d3de60] [c0141794] __vmalloc_node+0x44/0x54
> [c8d3df80] [c0c06510] init_IRQ+0x34/0xd4
> [c8d3dfa0] [c0c03440] start_kernel+0x424/0x558
> [c8d3dff0] [00003540] 0x3540
> Code: 39490999 7d4901a4 39290aaa 7d2a01a4 4c00012c 4bffff20 9421ffe0 7c08002a6 3d20c0d4 93e1001c 90010024 83e95278 <943f1ff0> 7fe1fb78 48840c6d 80210000
> ---[ end trace 0000000000000000 ]---
>
> Kernel panic - not syncing: Attempted to kill the idle task!
> Rebooting in 48 seconds..

This looks very close to the crash a few weeks ago which bisected to the
same commit [1].

Can you try applying this fix [2] which is on its way upstream?

[1] https://lore.kernel.org/linux-mm/3f86d58e-7f36-c6b4-c43a-2a7bcffd3bd@xxxxxxxxxxxxxx/
[2] https://lore.kernel.org/lkml/20230915174444.2835306-1-Liam.Howlett@xxxxxxxxxx/

>
>
> The bisect revealed this commit:
> # git bisect good
> cfeb6ae8bcb96ccf674724f223661bbcef7b0d0b is the first bad commit
> commit cfeb6ae8bcb96ccf674724f223661bbcef7b0d0b
> Author: Liam R. Howlett <Liam.Howlett@xxxxxxxxxx>
> Date: Fri Aug 18 20:43:55 2023 -0400
>
> maple_tree: disable mas_wr_append() when other readers are possible
>
> The current implementation of append may cause duplicate data and/or
> incorrect ranges to be returned to a reader during an update. Although
> this has not been reported or seen, disable the append write operation
> while the tree is in rcu mode out of an abundance of caution.
>
> During the analysis of the mas_next_slot() the following was
> artificially created by separating the writer and reader code:
>
> Writer: reader:
> mas_wr_append
> set end pivot
> updates end metata
> Detects write to last slot
> last slot write is to start of slot
> store current contents in slot
> overwrite old end pivot
> mas_next_slot():
> read end metadata
> read old end pivot
> return with incorrect range
> store new value
>
> Alternatively:
>
> Writer: reader:
> mas_wr_append
> set end pivot
> updates end metata
> Detects write to last slot
> last lost write to end of slot
> store value
> mas_next_slot():
> read end metadata
> read old end pivot
> read new end pivot
> return with incorrect range
> set old end pivot
>
> There may be other accesses that are not safe since we are now updating
> both metadata and pointers, so disabling append if there could be rcu
> readers is the safest action.
>
> Link: https://lkml.kernel.org/r/20230819004356.1454718-2-Liam.Howlett@xxxxxxxxxx
> Fixes: 54a611b60590 ("Maple Tree: add new data structure")
> Signed-off-by: Liam R. Howlett <Liam.Howlett@xxxxxxxxxx>
> Cc: <stable@xxxxxxxxxxxxxxx>
> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
>
> lib/maple_tree.c | 7 +++++++
> 1 file changed, 7 insertions(+)
>
>
> And indeed when I revert commit cfeb6ae8bcb96ccf674724f223661bbcef7b0d0b kernel 6.5.5 succeeds booting with CONFIG_VMAP_STACK=y enabled. dmesg of the successful boot with the reverted commit attached, also kernel .config and the bisect.log.
>
> Regards,
> Erhard F.