[CHERRY-PICK] UefiCpuPkg: x86 CpuDxe: Allocate AP Exception Stack Below 4GB #1246
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…ow 4GB When setting up the APs' exception stacks, the x86 CpuDxe allocates any range and then copies over the existing GDT and IDT and adds the appropriate new entries for this AP, then installs them. This can cause an issue if the allocated buffer is over 4GB because the next time the AP is started, it goes through an INIT-SIPI-SIPI, stepping through real mode -> protected mode -> long mode and when it is in protected mode it needs a 32 code segment descriptor or else it will fault when trying to execute. If the GDT lives above 4GB, it cannot be accessed by the protected mode code and the triple fault is seen. This patch updates CpuDxe's MP management code to allocate the exception stacks for all APs below 4GB explicitly to avoid this problem, such as it does with the BSP's GDT that first gets populated to the APs. Signed-off-by: Oliver Smith-Denny <[email protected]>
makubacki
approved these changes
Dec 18, 2024
apop5
approved these changes
Dec 18, 2024
vincent-j-zimmer
approved these changes
Dec 18, 2024
Codecov ReportAll modified and coverable lines are covered by tests ✅
Additional details and impacted files@@ Coverage Diff @@
## dev/202405 #1246 +/- ##
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- Coverage 1.60% 1.60% -0.01%
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Files 1377 1377
Lines 359246 359259 +13
Branches 5524 5524
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Hits 5760 5760
- Misses 353379 353392 +13
Partials 107 107
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Description
When setting up the APs' exception stacks, the x86 CpuDxe allocates any range and then copies over the existing GDT and IDT and adds the appropriate new entries for this AP, then installs them.
This can cause an issue if the allocated buffer is over 4GB because the next time the AP is started, it goes through an INIT-SIPI-SIPI, stepping through real mode -> protected mode -> long mode and when it is in protected mode it needs a 32 code segment descriptor or else it will fault when trying to execute. If the GDT lives above 4GB, it cannot be accessed by the protected mode code and the triple fault is seen.
This patch updates CpuDxe's MP management code to allocate the exception stacks for all APs below 4GB explicitly to avoid this problem, such as it does with the BSP's GDT that first gets populated to the APs.
Cherry-picked from: tianocore/edk2@924780f
How This Was Tested
Tested on a platform that had allocated the AP exception stacks above 4GB and so triple faulted. It booted after this patch.
Integration Instructions
N/A.