Commit 5944ce092b97 ("arch_topology: Build cacheinfo from primary CPU")
adds functionality that architectures can use to optionally allocate and
build cacheinfo early during boot. Commit 6539cffa9495 ("cacheinfo: Add
arch specific early level initializer") lets secondary CPUs correct (and
reallocate memory) cacheinfo data if needed.

If the early build functionality is not used and cacheinfo does not need
correction, memory for cacheinfo is never allocated. x86 does not use the
early build functionality. Consequently, during the cacheinfo CPU hotplug
callback, last_level_cache_is_valid() attempts to dereference a NULL
pointer:

     BUG: kernel NULL pointer dereference, address: 0000000000000100
     #PF: supervisor read access in kernel mode
     #PF: error_code(0x0000) - not present page
     PGD 0 P4D 0
     Oops: 0000 [#1] PREEPMT SMP NOPTI
     CPU: 0 PID 19 Comm: cpuhp/0 Not tainted 6.4.0-rc2 #1
     RIP: 0010: last_level_cache_is_valid+0x95/0xe0a

Allocate memory for cacheinfo during the cacheinfo CPU hotplug callback if
not done earlier.

Moreover, before determining the validity of the last-level cache info,
ensure that it has been allocated. Simply checking for non-zero
cache_leaves() is not sufficient, as some architectures (e.g., Intel
processors) have non-zero cache_leaves() before allocation.

Dereferencing NULL cacheinfo can occur in update_per_cpu_data_slice_size().
This function iterates over all online CPUs. However, a CPU may have come
online recently, but its cacheinfo may not have been allocated yet.

While here, remove an unnecessary indentation in allocate_cache_info().

Reviewed-by: Andreas Herrmann <aherrmann@suse.de>
Reviewed-by: Nikolay Borisov <nik.borisov@suse.com>
Reviewed-by: Radu Rendec <rrendec@redhat.com>
Reviewed-by: Sudeep Holla <sudeep.holla@arm.com>
Tested-by: Andreas Herrmann <aherrmann@suse.de>
Fixes: 6539cffa9495 ("cacheinfo: Add arch specific early level initializer")
Signed-off-by: Ricardo Neri <ricardo.neri-calderon@linux.intel.com>
---
Cc: Andreas Herrmann <aherrmann@suse.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Chen Yu <yu.c.chen@intel.com>
Cc: Huang Ying <ying.huang@intel.com>
Cc: Len Brown <len.brown@intel.com>
Cc: Nikolay Borisov <nik.borisov@suse.com>
Cc: Radu Rendec <rrendec@redhat.com>
Cc: Pierre Gondois <Pierre.Gondois@arm.com>
Cc: Pu Wen <puwen@hygon.cn>
Cc: "Rafael J. Wysocki" <rafael.j.wysocki@intel.com>
Cc: Sudeep Holla <sudeep.holla@arm.com>
Cc: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
Cc: Will Deacon <will@kernel.org>
Cc: Zhang Rui <rui.zhang@intel.com>
Cc: linux-arm-kernel@lists.infradead.org
Cc: stable@vger.kernel.org # 6.3+
---
Changes since v6:
 * Merged patches 1 and 2 of v6 into one. (Borislav)
 * Merged the history of patches 1 and 2 into this patch.
 * Kept the Reviewed-by and Tested-by tags from the two merged patches.
 * Fixed a formatting issue in allocate_cache_info(). (Borislav)

Changes since v5:
 * Fixed nonsensical subject (Nikolay).
 * Added Reviewed-by and Tested-by tags from Andreas. Thanks!
 * Added Reviewed-by tag from Nikolay. Thanks!

Changes since v4:
 * Combined checks for per_cpu_cacheinfo() and cache_leaves() in a single
   line. (Sudeep)
 * Added Reviewed-by tag from Sudeep. Thanks!

Changes since v3:
 * Added Reviewed-by tag from Radu and Sudeep. Thanks!

Changes since v2:
 * Introduced this patch.

Changes since v1:
 * N/A

---
The motivation for commit 5944ce092b97 was to prevent a BUG splat in
PREEMPT_RT kernels during memory allocation. This splat is not observed on
x86 because the memory allocation for cacheinfo happens in
detect_cache_attributes() from the cacheinfo CPU hotplug callback.

The dereference of a NULL cacheinfo is not observed today because
cache_leaves(cpu) is zero until after init_cache_level() is called
(during the CPU hotplug callback). A subsequent changeset will set
the number of cache leaves earlier and the NULL-pointer dereference
will be observed.
---
 drivers/base/cacheinfo.c | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/drivers/base/cacheinfo.c b/drivers/base/cacheinfo.c
index 23b8cba4a2a3..5f35a76cba2a 100644
--- a/drivers/base/cacheinfo.c
+++ b/drivers/base/cacheinfo.c
@@ -58,7 +58,7 @@ bool last_level_cache_is_valid(unsigned int cpu)
 {
 	struct cacheinfo *llc;
 
-	if (!cache_leaves(cpu))
+	if (!cache_leaves(cpu) || !per_cpu_cacheinfo(cpu))
 		return false;
 
 	llc = per_cpu_cacheinfo_idx(cpu, cache_leaves(cpu) - 1);
@@ -481,8 +481,7 @@ int __weak populate_cache_leaves(unsigned int cpu)
 static inline
 int allocate_cache_info(int cpu)
 {
-	per_cpu_cacheinfo(cpu) = kcalloc(cache_leaves(cpu),
-					 sizeof(struct cacheinfo), GFP_ATOMIC);
+	per_cpu_cacheinfo(cpu) = kcalloc(cache_leaves(cpu), sizeof(struct cacheinfo), GFP_ATOMIC);
 	if (!per_cpu_cacheinfo(cpu)) {
 		cache_leaves(cpu) = 0;
 		return -ENOMEM;
@@ -554,7 +553,11 @@ static inline int init_level_allocate_ci(unsigned int cpu)
 	 */
 	ci_cacheinfo(cpu)->early_ci_levels = false;
 
-	if (cache_leaves(cpu) <= early_leaves)
+	/*
+	 * Some architectures (e.g., x86) do not use early initialization.
+	 * Allocate memory now in such case.
+	 */
+	if (cache_leaves(cpu) <= early_leaves && per_cpu_cacheinfo(cpu))
 		return 0;
 
 	kfree(per_cpu_cacheinfo(cpu));
-- 
2.34.1

On Fri, Sep 13, 2024 at 01:31:53AM -0700, Ricardo Neri wrote:
> The motivation for commit 5944ce092b97 was to prevent a BUG splat in
> PREEMPT_RT kernels during memory allocation. This splat is not observed on
> x86 because the memory allocation for cacheinfo happens in
> detect_cache_attributes() from the cacheinfo CPU hotplug callback.
> 
> The dereference of a NULL cacheinfo is not observed today because
> cache_leaves(cpu) is zero until after init_cache_level() is called
> (during the CPU hotplug callback). A subsequent changeset will set
> the number of cache leaves earlier and the NULL-pointer dereference
> will be observed.

Lemme get this straight: this NULL ptr deref will happen after your changes so
we don't really need this patch in stable and thus no Fixes: tag at all,
right?

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette

On Tue, Oct 08, 2024 at 05:51:06PM +0200, Borislav Petkov wrote:
> On Fri, Sep 13, 2024 at 01:31:53AM -0700, Ricardo Neri wrote:
> > The motivation for commit 5944ce092b97 was to prevent a BUG splat in
> > PREEMPT_RT kernels during memory allocation. This splat is not observed on
> > x86 because the memory allocation for cacheinfo happens in
> > detect_cache_attributes() from the cacheinfo CPU hotplug callback.
> > 
> > The dereference of a NULL cacheinfo is not observed today because
> > cache_leaves(cpu) is zero until after init_cache_level() is called
> > (during the CPU hotplug callback). A subsequent changeset will set
> > the number of cache leaves earlier and the NULL-pointer dereference
> > will be observed.
> 
> Lemme get this straight: this NULL ptr deref will happen after your changes

Yes. It does happen only after my changes.

> so we don't really need this patch in stable and thus no Fixes: tag at all,
> right?

IMHO, this patchset is needed in stable kernel because /sys/devices/system/cpu
cpu0/cache would be broken on Meteor Lake and processors like it.

This patch alone is not needed in stable kernels.

Thanks and BR,
Ricardo

On Tue, Oct 08, 2024 at 10:00:15AM -0700, Ricardo Neri wrote:
> IMHO, this patchset is needed in stable kernel because
> /sys/devices/system/cpu cpu0/cache would be broken on Meteor Lake and
> processors like it.

Is Meteor Lake and processors like it an already released product?

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette

On Tue, Oct 08, 2024 at 08:41:45PM +0200, Borislav Petkov wrote:
> On Tue, Oct 08, 2024 at 10:00:15AM -0700, Ricardo Neri wrote:
> > IMHO, this patchset is needed in stable kernel because
> > /sys/devices/system/cpu cpu0/cache would be broken on Meteor Lake and
> > processors like it.
> 
> Is Meteor Lake and processors like it an already released product?

Yes, Meteor Lake was released in December 2023. It can be found in various
laptops.