x86: Fix .bss corruption

- Report

The following highly indeterministic kernel panic was reported to occur every few hundred boots:

    Kernel panic - not syncing: Fatal exception
     RIP: 0010:alloc_ucounts+0x68/0x280
     RSP: 0018:ffffb53ac13dfe98 EFLAGS: 00010006
     RAX: 0000000000000000 RBX: 000000015b804063 RCX: ffff9bb60d5aa500
     RDX: 0000000000000001 RSI: 00000000000003fb RDI: 0000000000000001
     RBP: ffffb53ac13dfec0 R08: 0000000000000000 R09: ffffd53abf600000
     R10: ffff9bb60b4bdd80 R11: ffffffffbcde7bb0 R12: ffffffffbf04e710
     R13: ffffffffbf405160 R14: 00000000000003fb R15: ffffffffbf1afc60
     FS:  00007f5be44d5000(0000) GS:ffff9bb6b9cc0000(0000) knlGS:0000000000000000
     CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
     CR2: 000000015b80407b CR3: 00000001038ea000 CR4: 00000000000406a0
     DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
     DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
     Call Trace:
      <TASK>
      __sys_setuid+0x1a0/0x290
      __x64_sys_setuid+0xc/0x10
      do_syscall_64+0x43/0x90
      ? asm_exc_page_fault+0x8/0x30
      entry_SYSCALL_64_after_hwframe+0x44/0xae

- Problem

The issue was eventually tracked down to the attempted dereference of a value located in a corrupted hash table. ucounts_hashtable is an array of 1024 struct hlists. Each element is the head of its own linked list where previous ucount allocations are stored. The [20]th element of ucounts_hashtable was being consistently trashed on each and every boot. However the indeterminism comes from it being accessed only every few hundred boots.

The issue disappeared, or was at least unidentifiable when !(LTO=full) or when memory base randomisation (a.k.a. KASLR) was disabled, rending GDB all but impossible to use effectively.

The cause of the corruption was uncovered using a verity of different debugging techniques and was eventually tracked down to page table manipulation in early architecture setup.

The following line in arch/x86/realmode/init.c [0] allocates a variable, just 8 Bytes in size, to "hold the pgd entry used on booting additional CPUs":

pgd_t trampoline_pgd_entry;

The address of that variable is then passed from init_trampoline_kaslr() via a call to set_pgd() [1] to have a value (not too important here) assigned to it. Numerous abstractions take place, eventually leading to native_set_p4d(), an inline function [2] contained in arch/x86/include/asm/pgtable_64.h.

From here, intentionally or otherwise, a call to pti_set_user_pgtbl() is made. This is where the out-of-bounds write eventually occurs. It is not known (by me) why this function is called. The returned result is subsequently used as a value to write using the WRITE_ONCE macro. Perhaps the premature write is not intended. This is what I hope to find out.

A little way down in pti_set_user_pgtbl() [3] the following line occurs:

kernel_to_user_pgdp(pgdp)->pgd = pgd.pgd

The kernel_to_user_pgdp() part takes the address of pgdp (a.k.a. trampoline_pgd_entry) and ends up flipping the 12th bit, essentially adding 4k (0x1000) to the address. Then the part at the end assigns our value (still not important here) to it. However, if we remember that only 8 Bytes was allocated (globally) for trampoline_pgd_entry, then means we just stored the value into the outlands (what we now know to be allocated to another global storage user ucounts_hashtable).

[0] https://elixir.bootlin.com/linux/latest/source/arch/x86/realmode/init.c#L18
[1] https://elixir.bootlin.com/linux/latest/source/arch/x86/mm/kaslr.c#L178
[2] https://elixir.bootlin.com/linux/latest/source/arch/x86/include/asm/pgtable_64.h#L142
[3] https://elixir.bootlin.com/linux/latest/source/arch/x86/mm/pti.c#L142

Lee Jones (1):
x86/mm/KASLR: Store pud_page_tramp into entry rather than page

 arch/x86/mm/kaslr.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

2.41.0.162.gfafddb0af9-goog

set_pgd() expects to be passed whole pages to operate on, whereas trampoline_pgd_entry is, as the name suggests, an entry. The ramifications for using set_pgd() here are that the following thread of execution will not only place the suggested value into the trampoline_pgd_entry (8-Byte globally stored [.bss]) variable, PTI will also attempt to replicate that value into the non-existent neighboring user page (located +4k away), leading to the corruption of other global [.bss] stored variables.

---
 arch/x86/mm/kaslr.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

index 557f0fe25dff4..ee6f22f3385b6 100644
@@ -175,7 +175,7 @@ void __meminit init_trampoline_kaslr(void)
 		set_pgd(&trampoline_pgd_entry,
 			__pgd(_KERNPG_TABLE | __pa(p4d_page_tramp)));
 	} else {
-		set_pgd(&trampoline_pgd_entry,
-			__pgd(_KERNPG_TABLE | __pa(pud_page_tramp)));
+		trampoline_pgd_entry =
+			__pgd(_KERNPG_TABLE | __pa(pud_page_tramp));
 	}
 }

2.41.0.162.gfafddb0af9-goog