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

[Mat Martineau <mathew.j.martineau@linux.intel.com>]

On Thu, 21 Apr 2022, 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 [1]
>  - wait ~1 minute
>  - start a kmemleak scan
>
> It seems kmemleak has an issue with the array allocated in
> msr_build_context() and assigned to a pointer in a static structure
> (saved_context.saved_msrs->array): there is no leak then.
>
> It looks like this is a limitation from kmemleak but that's alright,
> kmemleak_no_leak() can be used to avoid complaining about that.
>
> 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").
>
> Depending on their CPUs, others have probably 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. But I
> understand if someone says the origin of this issue is more on
> kmemleak's side. What is unclear to me is why this issue was not seen by
> other people and CIs. Maybe the kernel config [1]?
>
> [1] https://github.com/multipath-tcp/mptcp_net-next/files/8531660/kmemleak-cpu-sched-bisect.kconfig.txt
>

Hi Matthieu -

It looks like the root cause here is alignment within the packed struct 
saved_context (from suspend_64.h). Kmemleak only searches for pointers 
that are aligned, but pahole shows that the saved_msrs struct member and 
all members after it in the structure are unaligned:

(gcc 11.2.1, x86_64)

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 odd offset values for the remainder of this struct    vvv       */

 	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__));

If I move 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 and __save/__restore_processor_state() if the struct is 
modified - looks like it's the members before misc_enable that must be 
carefully placed.

So far I've only tried this on my local machine, I'll work on getting more 
thorough validation.

Looks like struct saved_context in suspend_32.h has similar odd alignment.


- Mat


> 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
> Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
> ---
> arch/x86/power/cpu.c | 4 ++++
> 1 file changed, 4 insertions(+)
>
> diff --git a/arch/x86/power/cpu.c b/arch/x86/power/cpu.c
> index 3822666fb73d..1467c6d1a966 100644
> --- a/arch/x86/power/cpu.c
> +++ b/arch/x86/power/cpu.c
> @@ -14,6 +14,7 @@
> #include <linux/tboot.h>
> #include <linux/dmi.h>
> #include <linux/pgtable.h>
> +#include <linux/kmemleak.h>
>
> #include <asm/proto.h>
> #include <asm/mtrr.h>
> @@ -413,6 +414,9 @@ static int msr_build_context(const u32 *msr_id, const int num)
> 		return -ENOMEM;
> 	}
>
> +	/* The pointer is going to be stored in static struct (saved_context) */
> +	kmemleak_not_leak(msr_array);
> +
> 	if (saved_msrs->array) {
> 		/*
> 		 * Multiple callbacks can invoke this function, so copy any
> -- 
> 2.34.1
>
>

--
Mat Martineau
Intel