Re: [PATCH v1.1] xen/commom/dt-overlay: Fix missing lock when remove the device

[Julien Grall <julien@xxxxxxx>]

Hi Henry,

On 26/04/2024 02:55, Henry Wang wrote:
> If CONFIG_DEBUG=y, below assertion will be triggered:
> (XEN) Assertion 'rw_is_locked(&dt_host_lock)' failed at 
> drivers/passthrough/device_tree.c:146
> (XEN) ----[ Xen-4.19-unstable  arm64  debug=y  Not tainted ]----
> (XEN) CPU:    0
> (XEN) PC:     00000a0000257418 iommu_remove_dt_device+0x8c/0xd4
> (XEN) LR:     00000a00002573a0
> (XEN) SP:     00008000fff7fb30
> (XEN) CPSR:   0000000000000249 MODE:64-bit EL2h (Hypervisor, handler)
> [...]
>
> (XEN) Xen call trace:
> (XEN)    [<00000a0000257418>] iommu_remove_dt_device+0x8c/0xd4 (PC)
> (XEN)    [<00000a00002573a0>] iommu_remove_dt_device+0x14/0xd4 (LR)
> (XEN)    [<00000a000020797c>] dt-overlay.c#remove_node_resources+0x8c/0x90
> (XEN)    [<00000a0000207f14>] dt-overlay.c#remove_nodes+0x524/0x648
> (XEN)    [<00000a0000208460>] dt_overlay_sysctl+0x428/0xc68
> (XEN)    [<00000a00002707f8>] arch_do_sysctl+0x1c/0x2c
> (XEN)    [<00000a0000230b40>] do_sysctl+0x96c/0x9ec
> (XEN)    [<00000a0000271e08>] traps.c#do_trap_hypercall+0x1e8/0x288
> (XEN)    [<00000a0000273490>] do_trap_guest_sync+0x448/0x63c
> (XEN)    [<00000a000025c480>] entry.o#guest_sync_slowpath+0xa8/0xd8
> (XEN)
> (XEN)
> (XEN) ****************************************
> (XEN) Panic on CPU 0:
> (XEN) Assertion 'rw_is_locked(&dt_host_lock)' failed at 
> drivers/passthrough/device_tree.c:146
> (XEN) ****************************************
>
> This is because iommu_remove_dt_device() is called without taking the
> dt_host_lock. Fix the issue by taking and releasing the lock properly.
>
> Fixes: 7e5c4a8b86f1 ("xen/arm: Implement device tree node removal 
> functionalities")
> Signed-off-by: Henry Wang <xin.wang2@xxxxxxx>
> ---
> v1.1:
> - Move the unlock position before the check of rc.
> ---
>   xen/common/dt-overlay.c | 2 ++
>   1 file changed, 2 insertions(+)
>
> diff --git a/xen/common/dt-overlay.c b/xen/common/dt-overlay.c
> index 1b197381f6..ab8f43aea2 100644
> --- a/xen/common/dt-overlay.c
> +++ b/xen/common/dt-overlay.c
> @@ -381,7 +381,9 @@ static int remove_node_resources(struct dt_device_node 
> *device_node)
>       {
>           if ( dt_device_is_protected(device_node) )
>           {
> +            write_lock(&dt_host_lock);
Looking at the code, we are not modifying the device_node, so shouldn't 
this be a read_lock()?
That said, even though either fix your issue, I am not entirely 
convinced this is the correct position for the lock. From my 
understanding, dt_host_lock is meant to ensure that the DT node will not 
disappear behind your back. So in theory, shouldn't the lock be taken as 
soon as you get hold of device_node?
Cheers,

--
Julien Grall