Re: [PATCH v2] x86/pm: fix false positive kmemleak report in msr_build_context()

["Rafael J. Wysocki" <rafael.j.wysocki@intel.com>]

On 4/23/2022 8:24 PM, Matthieu Baerts wrote:
> Since commit e2a1256b17b1 ("x86/speculation: Restore speculation related MSRs during S3 resume"),
> kmemleak reports this issue:
>
>    unreferenced object 0xffff888009cedc00 (size 256):
>      comm "swapper/0", pid 1, jiffies 4294693823 (age 73.764s)
>      hex dump (first 32 bytes):
>        00 00 00 00 00 00 00 00 48 00 00 00 00 00 00 00  ........H.......
>        00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
>      backtrace:
>        msr_build_context (include/linux/slab.h:621)
>        pm_check_save_msr (arch/x86/power/cpu.c:520)
>        do_one_initcall (init/main.c:1298)
>        kernel_init_freeable (init/main.c:1370)
>        kernel_init (init/main.c:1504)
>        ret_from_fork (arch/x86/entry/entry_64.S:304)
>
> It is easy to reproduce it on my side:
>
>    - boot the VM with a debug kernel config (see the 'Closes:' tag)
>    - wait ~1 minute
>    - start a kmemleak scan
>
> It seems kmemleak has an issue with the array allocated in
> msr_build_context(). This array is assigned to a pointer in a static
> structure (saved_context.saved_msrs->array): there is no leak then.
>
> A simple fix for this issue would be to use kmemleak_no_leak() but Mat
> noticed that the root cause here is alignment within the packed 'struct
> saved_context' (from suspend_64.h). Kmemleak only searches for pointers
> that are aligned (see how pointers are scanned in kmemleak.c), but
> pahole shows that the saved_msrs struct member and all members after it
> in the structure are unaligned:
>
>    struct saved_context {
>      struct pt_regs             regs;                 /*     0   168 */
>      /* --- cacheline 2 boundary (128 bytes) was 40 bytes ago --- */
>      u16                        ds;                   /*   168     2 */
>      u16                        es;                   /*   170     2 */
>      u16                        fs;                   /*   172     2 */
>      u16                        gs;                   /*   174     2 */
>      long unsigned int          kernelmode_gs_base;   /*   176     8 */
>      long unsigned int          usermode_gs_base;     /*   184     8 */
>      /* --- cacheline 3 boundary (192 bytes) --- */
>      long unsigned int          fs_base;              /*   192     8 */
>      long unsigned int          cr0;                  /*   200     8 */
>      long unsigned int          cr2;                  /*   208     8 */
>      long unsigned int          cr3;                  /*   216     8 */
>      long unsigned int          cr4;                  /*   224     8 */
>      u64                        misc_enable;          /*   232     8 */
>      bool                       misc_enable_saved;    /*   240     1 */
>
>     /* Note below odd offset values for the remainder of this struct */
>
>      struct saved_msrs          saved_msrs;           /*   241    16 */
>      /* --- cacheline 4 boundary (256 bytes) was 1 bytes ago --- */
>      long unsigned int          efer;                 /*   257     8 */
>      u16                        gdt_pad;              /*   265     2 */
>      struct desc_ptr            gdt_desc;             /*   267    10 */
>      u16                        idt_pad;              /*   277     2 */
>      struct desc_ptr            idt;                  /*   279    10 */
>      u16                        ldt;                  /*   289     2 */
>      u16                        tss;                  /*   291     2 */
>      long unsigned int          tr;                   /*   293     8 */
>      long unsigned int          safety;               /*   301     8 */
>      long unsigned int          return_address;       /*   309     8 */
>
>      /* size: 317, cachelines: 5, members: 25 */
>      /* last cacheline: 61 bytes */
>    } __attribute__((__packed__));
>
> By moving 'misc_enable_saved' to the end of the struct declaration,
> 'saved_msrs' fits in before the cacheline 4 boundary and the kmemleak
> warning goes away.
>
> The comment above the 'saved_context' declaration says to check
> wakeup_64.S file and __save/__restore_processor_state() if the struct is
> modified: it looks like it's the members before 'misc_enable' that must
> be carefully placed.
>
> At the end, the false positive kmemleak report is due to a limitation
> from kmemleak but that's always good to avoid unaligned member for
> optimisation purposes.
>
> Please note that it looks like this issue is not new, e.g.
>
>    https://lore.kernel.org/all/9f1bb619-c4ee-21c4-a251-870bd4db04fa@lwfinger.net/
>    https://lore.kernel.org/all/94e48fcd-1dbd-ebd2-4c91-f39941735909@molgen.mpg.de/
>
> But on my side, msr_build_context() is only used since:
>
>    commit e2a1256b17b1 ("x86/speculation: Restore speculation related MSRs during S3 resume").
>
> Others probably have the same issue since:
>
>    commit 7a9c2dd08ead ("x86/pm: Introduce quirk framework to save/restore extra MSR registers around suspend/resume"),
>
> Hence the 'Fixes' tag here below to help with the backports.
>
> Fixes: 7a9c2dd08ead ("x86/pm: Introduce quirk framework to save/restore extra MSR registers around suspend/resume")
> Closes: https://github.com/multipath-tcp/mptcp_net-next/issues/268
> Suggested-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
> Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>

All good AFAICS.

Reviewed-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>


> ---
>   arch/x86/include/asm/suspend_32.h |  2 +-
>   arch/x86/include/asm/suspend_64.h | 12 ++++++++----
>   2 files changed, 9 insertions(+), 5 deletions(-)
>
> diff --git a/arch/x86/include/asm/suspend_32.h b/arch/x86/include/asm/suspend_32.h
> index 7b132d0312eb..a800abb1a992 100644
> --- a/arch/x86/include/asm/suspend_32.h
> +++ b/arch/x86/include/asm/suspend_32.h
> @@ -19,7 +19,6 @@ struct saved_context {
>   	u16 gs;
>   	unsigned long cr0, cr2, cr3, cr4;
>   	u64 misc_enable;
> -	bool misc_enable_saved;
>   	struct saved_msrs saved_msrs;
>   	struct desc_ptr gdt_desc;
>   	struct desc_ptr idt;
> @@ -28,6 +27,7 @@ struct saved_context {
>   	unsigned long tr;
>   	unsigned long safety;
>   	unsigned long return_address;
> +	bool misc_enable_saved;
>   } __attribute__((packed));
>   
>   /* routines for saving/restoring kernel state */
> diff --git a/arch/x86/include/asm/suspend_64.h b/arch/x86/include/asm/suspend_64.h
> index 35bb35d28733..bb7023dbf524 100644
> --- a/arch/x86/include/asm/suspend_64.h
> +++ b/arch/x86/include/asm/suspend_64.h
> @@ -14,9 +14,13 @@
>    * Image of the saved processor state, used by the low level ACPI suspend to
>    * RAM code and by the low level hibernation code.
>    *
> - * If you modify it, fix arch/x86/kernel/acpi/wakeup_64.S and make sure that
> - * __save/__restore_processor_state(), defined in arch/x86/kernel/suspend_64.c,
> - * still work as required.
> + * If you modify it before 'misc_enable', fix arch/x86/kernel/acpi/wakeup_64.S
> + * and make sure that __save/__restore_processor_state(), defined in
> + * arch/x86/kernel/suspend_64.c, still work as required.
> + *
> + * Because the structure is packed, make sure to avoid unaligned members. For
> + * optimisations purposes but also because tools like Kmemleak only search for
> + * pointers that are aligned.
>    */
>   struct saved_context {
>   	struct pt_regs regs;
> @@ -36,7 +40,6 @@ struct saved_context {
>   
>   	unsigned long cr0, cr2, cr3, cr4;
>   	u64 misc_enable;
> -	bool misc_enable_saved;
>   	struct saved_msrs saved_msrs;
>   	unsigned long efer;
>   	u16 gdt_pad; /* Unused */
> @@ -48,6 +51,7 @@ struct saved_context {
>   	unsigned long tr;
>   	unsigned long safety;
>   	unsigned long return_address;
> +	bool misc_enable_saved;
>   } __attribute__((packed));
>   
>   #define loaddebug(thread,register) \