Read Disk does a modification of ExtMbrStartingLba with the code MultU64x32 (ExtMbrStartingLba, BlockSize) Error detection to see if ExtMbrStartingLBA has a value of 0. This is invalid as LBA 0 = MBR. After modification, the next time ExtMbrStartingLba is in this function if ExtMbrStartingLba is set to 0 in the MBR it never passes the while/do evaluation It is multiplied by 0 by read disk , set to 0 by an invalid MBR and goes back to evaluation This condition will also cause Ws19 and WS22 to hang, however Microsoft has developed a hotfix patch that will be released in 2022 Cc: Liming Gao > Cc: Jian J Wang > Cc: Hao A Wu > Cc: Ray Ni > Cc: Zhichao Gao > Signed-off-by: Craig Edwards > Date: Wed Jan 5 12:27:46 2022 -0600 On branch graceful_handle_mbr_hang_edit1 Changes to be committed: modified: MdeModulePkg/Universal/Disk/PartitionDxe/Mbr.c --- MdeModulePkg/Universal/Disk/PartitionDxe/Mbr.c | 6 ++++++ 1 file changed, 6 insertions(+) diff --git a/MdeModulePkg/Universal/Disk/PartitionDxe/Mbr.c b/MdeModulePkg/Universal/Disk/PartitionDxe/Mbr.c index 0f8dc5486521..ad18840e5efd 100644 --- a/MdeModulePkg/Universal/Disk/PartitionDxe/Mbr.c +++ b/MdeModulePkg/Universal/Disk/PartitionDxe/Mbr.c @@ -293,6 +293,12 @@ PartitionInstallMbrChildHandles ( (Mbr->Partition[0].OSIndicator == EXTENDED_WINDOWS_PARTITION)) { ExtMbrStartingLba = UNPACK_UINT32 (Mbr->Partition[0].StartingLBA); + // + // A value of 0 is invalid for StartingLBA + // + if (ExtMbrStartingLba == 0) { + break; + } continue; } -- 2.32.0.windows.1 Craig Edwards Software Engineer Dell | GDP | PSE | COMMS | BIOS Craig_Edwards@Dell.com Internal Use - Confidential