CVE-2024-26688 fs,hugetlb: fix NULL pointer dereference in hugetlbs_fill_super
In the Linux kernel, the following vulnerability has been resolved:

fs,hugetlb: fix NULL pointer dereference in hugetlbs_fill_super

When configuring a hugetlb filesystem via the fsconfig() syscall, there is
a possible NULL dereference in hugetlbfs_fill_super() caused by assigning
NULL to ctx->hstate in hugetlbfs_parse_param() when the requested pagesize
is non valid.

E.g: Taking the following steps:

     fd = fsopen("hugetlbfs", FSOPEN_CLOEXEC);
     fsconfig(fd, FSCONFIG_SET_STRING, "pagesize", "1024", 0);
     fsconfig(fd, FSCONFIG_CMD_CREATE, NULL, NULL, 0);

Given that the requested "pagesize" is invalid, ctxt->hstate will be replaced
with NULL, losing its previous value, and we will print an error:

 ...
 ...
 case Opt_pagesize:
 ps = memparse(param->string, &rest);
 ctx->hstate = h;
 if (!ctx->hstate) {
         pr_err("Unsupported page size %lu MB\n", ps / SZ_1M);
         return -EINVAL;
 }
 return 0;
 ...
 ...

This is a problem because later on, we will dereference ctxt->hstate in
hugetlbfs_fill_super()

 ...
 ...
 sb->s_blocksize = huge_page_size(ctx->hstate);
 ...
 ...

Causing below Oops.

Fix this by replacing cxt->hstate value only when then pagesize is known
to be valid.

 kernel: hugetlbfs: Unsupported page size 0 MB
 kernel: BUG: kernel NULL pointer dereference, address: 0000000000000028
 kernel: #PF: supervisor read access in kernel mode
 kernel: #PF: error_code(0x0000) - not-present page
 kernel: PGD 800000010f66c067 P4D 800000010f66c067 PUD 1b22f8067 PMD 0
 kernel: Oops: 0000 [#1] PREEMPT SMP PTI
 kernel: CPU: 4 PID: 5659 Comm: syscall Tainted: G            E      6.8.0-rc2-default+ #22 5a47c3fef76212addcc6eb71344aabc35190ae8f
 kernel: Hardware name: Intel Corp. GROVEPORT/GROVEPORT, BIOS GVPRCRB1.86B.0016.D04.1705030402 05/03/2017
 kernel: RIP: 0010:hugetlbfs_fill_super+0xb4/0x1a0
 kernel: Code: 48 8b 3b e8 3e c6 ed ff 48 85 c0 48 89 45 20 0f 84 d6 00 00 00 48 b8 ff ff ff ff ff ff ff 7f 4c 89 e7 49 89 44 24 20 48 8b 03 <8b> 48 28 b8 00 10 00 00 48 d3 e0 49 89 44 24 18 48 8b 03 8b 40 28
 kernel: RSP: 0018:ffffbe9960fcbd48 EFLAGS: 00010246
 kernel: RAX: 0000000000000000 RBX: ffff9af5272ae780 RCX: 0000000000372004
 kernel: RDX: ffffffffffffffff RSI: ffffffffffffffff RDI: ffff9af555e9b000
 kernel: RBP: ffff9af52ee66b00 R08: 0000000000000040 R09: 0000000000370004
 kernel: R10: ffffbe9960fcbd48 R11: 0000000000000040 R12: ffff9af555e9b000
 kernel: R13: ffffffffa66b86c0 R14: ffff9af507d2f400 R15: ffff9af507d2f400
 kernel: FS:  00007ffbc0ba4740(0000) GS:ffff9b0bd7000000(0000) knlGS:0000000000000000
 kernel: CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
 kernel: CR2: 0000000000000028 CR3: 00000001b1ee0000 CR4: 00000000001506f0
 kernel: Call Trace:
 kernel:  <TASK>
 kernel:  ? __die_body+0x1a/0x60
 kernel:  ? page_fault_oops+0x16f/0x4a0
 kernel:  ? search_bpf_extables+0x65/0x70
 kernel:  ? fixup_exception+0x22/0x310
 kernel:  ? exc_page_fault+0x69/0x150
 kernel:  ? asm_exc_page_fault+0x22/0x30
 kernel:  ? __pfx_hugetlbfs_fill_super+0x10/0x10
 kernel:  ? hugetlbfs_fill_super+0xb4/0x1a0
 kernel:  ? hugetlbfs_fill_super+0x28/0x1a0
 kernel:  ? __pfx_hugetlbfs_fill_super+0x10/0x10
 kernel:  vfs_get_super+0x40/0xa0
 kernel:  ? __pfx_bpf_lsm_capable+0x10/0x10
 kernel:  vfs_get_tree+0x25/0xd0
 kernel:  vfs_cmd_create+0x64/0xe0
 kernel:  __x64_sys_fsconfig+0x395/0x410
 kernel:  do_syscall_64+0x80/0x160
 kernel:  ? syscall_exit_to_user_mode+0x82/0x240
 kernel:  ? do_syscall_64+0x8d/0x160
 kernel:  ? syscall_exit_to_user_mode+0x82/0x240
 kernel:  ? do_syscall_64+0x8d/0x160
 kernel:  ? exc_page_fault+0x69/0x150
 kernel:  entry_SYSCALL_64_after_hwframe+0x6e/0x76
 kernel: RIP: 0033:0x7ffbc0cb87c9
 kernel: Code: 00 90 90 90 90 90 90 90 90 90 90 90 90 90 90 66 90 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 8b 0d 97 96 0d 00 f7 d8 64 89 01 48
 kernel: RSP: 002b:00007ffc29d2f388 EFLAGS: 00000206 ORIG_RAX: 00000000000001af
 kernel: RAX: fffffffffff
---truncated---
CVSS v3.1 score: 5.5