CVE-2023-52489: In the Linux kernel, the following vulnerability has been resolved:mm/sparsemem: fix race in accessing memory_section->usageThe below race is observed on a PFN which falls into the device memoryregion with the system memory configuration where PFN's are such that[ZONE_NORMAL ZONE_DEVICE ZONE_NORMAL].  Since normal zone start and endpfn contains the device memory PFN's as well, the compaction triggeredwill try on the device memory PFN's too though they end up in NOP(becausepfn_to_online_page() returns NULL for ZONE_DEVICE memory sections).  Whenfrom other core, the section mappings are being removed for theZONE_DEVICE region, that the PFN in question belongs to, on whichcompaction is currently being operated is resulting into the kernel crashwith CONFIG_SPASEMEM_VMEMAP enabled.  The crash logs can be seen at [1].compact_zone()			memunmap_pages-------------			---------------__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 betweenthe pfn_valid()/pfn_section_valid() and the section deactivate withSPASEMEM_VMEMAP enabled.The commit b943f045a9af("mm/sparse: fix kernel crash withpfn_section_valid check") tried to address the same problem by clearingthe SECTION_HAS_MEM_MAP with the expectation of valid_section() returnsfalse 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) Free the ->usage with kfree_rcu() and set ms->usage = NULL.  No   attempt will be made to access ->usage after this as the   SECTION_HAS_MEM_MAP is cleared thus valid_section() return false.Thanks to David/Pavan for their inputs on this patch.[1] https://lore.kernel.org/linux-mm/994410bb-89aa-d987-1f50-f514903c55aa@quicinc.com/On Snapdragon SoC, with the mentioned memory configuration of PFN's as[ZONE_NORMAL ZONE_DEVICE ZONE_NORMAL], we are able to see bunch ofissues daily while testing on a device farm.For this particular issue below is the log.  Though the below log isnot directly pointing to the pfn_section_valid(){ ms->usage;}, when weloaded this dump on T32 lauterbach tool, it is pointing.[  540.578056] Unable to handle kernel NULL pointer dereference atvirtual address 0000000000000000[  540.578068] Mem abort info:[  540.578070]   ESR = 0x0000000096000005[  540.578073]   EC = 0x25: DABT (current EL), IL = 32 bits[  540.578077]   SET = 0, FnV = 0[  540.578080]   EA = 0, S1PTW = 0[  540.578082]   FSC = 0x05: level 1 translation fault[  540.578085] Data abort info:[  540.578086]   ISV = 0, ISS = 0x00000005[  540.578088]   CM = 0, WnR = 0[  540.579431] pstate: 82400005 (Nzcv daif +PAN -UAO +TCO -DIT -SSBSBTYPE=--)[  540.579436] pc : __pageblock_pfn_to_page+0x6c/0x14c[  540.579454] lr : compact_zone+0x994/0x1058[  540.579460] sp : ffffffc03579b510[  540.579463] x29: ffffffc03579b510 x28: 0000000000235800 x27:000000000000000c[  540.579470] x26: 0000000000235c00 x25: 0000000000000068 x24:ffffffc03579b640[  540.579477] x23: 0000000000000001 x22: ffffffc03579b660 x21:0000000000000000[  540.579483] x20: 0000000000235bff x19: ffffffdebf7e3940 x18:ffffffdebf66d140[  540.579489] x17: 00000000739ba063 x16: 00000000739ba063 x15:00000000009f4bff[  540.579495] x14: 0000008000000000 x13: 0000000000000000 x12:0000000000000001[  540.579501] x11: 0000000000000000 x10: 0000000000000000 x9 :ffffff897d2cd440[  540.579507] x8 : 0000000000000000 x7 : 0000000000000000 x6 :ffffffc03579b5b4[  540.579512] x5 : 0000000000027f25 x4 : ffffffc03579b5b8 x3 :0000000000000---truncated---
Key phrases: crash