Re: [PATCH] mm/sparsemem: fix race in accessing memory_section->usage

[Andrew Morton <akpm@linux-foundation.org>]

On Fri, 13 Oct 2023 18:34:27 +0530 Charan Teja Kalla <quic_charante@quicinc.com> wrote:

> The below race is observed on a PFN which falls into the device memory
> region with the system memory configuration where PFN's are such that
> [ZONE_NORMAL ZONE_DEVICE  ZONE_NORMAL]. Since normal zone start and
> end pfn contains the device memory PFN's as well, the compaction
> triggered will try on the device memory PFN's too though they end up in
> NOP(because pfn_to_online_page() returns NULL for ZONE_DEVICE memory
> sections). When from other core, the section mappings are being removed
> for the ZONE_DEVICE region, that the PFN in question belongs to,
> on which compaction is currently being operated is resulting into the
> kernel crash with CONFIG_SPASEMEM_VMEMAP enabled.

Seems this bug is four years old, yes?  It must be quite hard to hit.

When people review this, please offer opinions on whether a fix should
be backported into -stable kernels, thanks.

> compact_zone()			memunmap_page
> -------------			---------------
> __pageblock_pfn_to_page
>    ......
>  (a)pfn_valid():
>      valid_section()//return true
> 			      (b)__remove_pages()->
> 				  sparse_remove_section()->
> 				    section_deactivate():
> 				    [Free the array ms->usage and set
> 				     ms->usage = NULL]
>      pfn_section_valid()
>      [Access ms->usage which
>      is NULL]
> 
> NOTE: From the above it can be said that the race is reduced to between
> the pfn_valid()/pfn_section_valid() and the section deactivate with
> SPASEMEM_VMEMAP enabled.
> 
> The commit b943f045a9af("mm/sparse: fix kernel crash with
> pfn_section_valid check") tried to address the same problem by clearing
> the SECTION_HAS_MEM_MAP with the expectation of valid_section() returns
> false thus ms->usage is not accessed.
> 
> Fix this issue by the below steps:
> a) Clear SECTION_HAS_MEM_MAP before freeing the ->usage.
> b) RCU protected read side critical section will either return NULL when
> SECTION_HAS_MEM_MAP is cleared or can successfully access ->usage.
> c) Synchronize the rcu on the write side and free the ->usage. No
> attempt will be made to access ->usage after this as the
> SECTION_HAS_MEM_MAP is cleared thus valid_section() return false.
> 
> Since the section_deactivate() is a rare operation and will come in the
> hot remove path, impact of synchronize_rcu() should be negligble.
> 
> Fixes: f46edbd1b151 ("mm/sparsemem: add helpers track active portions of a section at boot")