x86/setup: consolidate early memory reservations

From: Mike Rapoport <[email protected]>

The early reservations of memory areas used by the firmware, bootloader, kernel text and data are spread over setup_arch(). Moreover, some of them happen *after* memblock allocations, e.g trim_platform_memory_ranges() and trim_low_memory_range() are called after reserve_real_mode() that allocates memory.

We did not observe corruption of these memory regions because memblock always allocates memory either from the end of memory (in top-down mode) or above the kernel image (in bottom-up mode). However, the bottom up mode is going to be updated to span the entire memory [1] to avoid limitations caused by KASLR.

Consolidate early memory reservations in a dedicated function to improve robustness against future changes. Having the early reservations in one place also makes it clearer what memory must be reserved before we allow memblock allocations.

[1] https://lore.kernel.org/lkml/[email protected]

Signed-off-by Mike Rapoport <[email protected]>

---
 arch/x86/kernel/setup.c | 80 ++++++++++++++++++++++-------------------
 1 file changed, 44 insertions(+), 36 deletions(-)

arch/x86/kernel/setup.c +44 -33

index 3412c4595efd..32cd2e790a0a 100644
@@ -728,7 +728,38 @@ static void __init trim_low_memory_range(void)
 	 */
 	memblock_reserve(0, ALIGN(reserve_low, PAGE_SIZE));
 }
-	
+
+static void __init early_reserve_memory(void)
+{
+	/*
+	 * Reserve the memory occupied by the kernel between _text and
+	 * __end_of_kernel_reserve symbols. Any kernel sections after the
+	 * __end_of_kernel_reserve symbol must be explicitly reserved with a
+	 * separate memblock_reserve() or they will be discarded.
+	 */
+	memblock_reserve(__pa_symbol(_text),
+			 (unsigned long)__end_of_kernel_reserve - (unsigned long)_text);
+
+	/*
+	 * Make sure page 0 is always reserved because on systems with
+	 * L1TF its contents can be leaked to user processes.
+	 */
+	memblock_reserve(0, PAGE_SIZE);
+
+	early_reserve_initrd();
+
+	if (efi_enabled(EFI_BOOT))
+		efi_memblock_x86_reserve_range();
+
+	memblock_x86_reserve_range_setup_data();
+
+	reserve_ibft_region();
+	reserve_bios_regions();
+
+	trim_platform_memory_ranges();
+	trim_low_memory_range();
+}
+
 /*
  * Dump out kernel offset information on panic.
  */

@@ -763,29 +794,6 @@ dump_kernel_offset(struct notifier_block *self, unsigned long v, void *p)
 
 void __init setup_arch(char **cmdline_p)
 {
-	/*
-	 * Reserve the memory occupied by the kernel between _text and
-	 * __end_of_kernel_reserve symbols. Any kernel sections after the
-	 * __end_of_kernel_reserve symbol must be explicitly reserved with a
-	 * separate memblock_reserve() or they will be discarded.
-	 */
-	memblock_reserve(__pa_symbol(_text),
-			 (unsigned long)__end_of_kernel_reserve - (unsigned long)_text);
-
-	/*
-	 * Make sure page 0 is always reserved because on systems with
-	 * L1TF its contents can be leaked to user processes.
-	 */
-	memblock_reserve(0, PAGE_SIZE);
-
-	early_reserve_initrd();
-
-	/*
-	 * At this point everything still needed from the boot loader
-	 * or BIOS or kernel text should be early reserved or marked not
-	 * RAM in e820. All other memory is free game.
-	 */
-
 #ifdef CONFIG_X86_32
 	memcpy(&boot_cpu_data, &new_cpu_data, sizeof(new_cpu_data));
 

@@ -909,8 +917,18 @@ void __init setup_arch(char **cmdline_p)
 
 	parse_early_param();
 
-	if (efi_enabled(EFI_BOOT))
-		efi_memblock_x86_reserve_range();
+	/*
+	 * Do some memory reservations *before* memory is added to
+	 * memblock, so memblock allocations won't overwrite it.
+	 * Do it after early param, so we could get (unlikely) panic from
+	 * serial.
+	 *
+	 * After this point everything still needed from the boot loader or
+	 * firmware or kernel text should be early reserved or marked not
+	 * RAM in e820. All other memory is free game.
+	 */
+	early_reserve_memory();
+
 #ifdef CONFIG_MEMORY_HOTPLUG
 	/*
 	 * Memory used by the kernel cannot be hot-removed because Linux

@@ -937,9 +955,6 @@ void __init setup_arch(char **cmdline_p)
 
 	x86_report_nx();
 
-	/* after early param, so could get panic from serial */
-	memblock_x86_reserve_range_setup_data();
-
 	if (acpi_mps_check()) {
 #ifdef CONFIG_X86_LOCAL_APIC
 		disable_apic = 1;

@@ -1031,8 +1046,6 @@ void __init setup_arch(char **cmdline_p)
 	 */
 	find_smp_config();
 
-	reserve_ibft_region();
-
 	early_alloc_pgt_buf();
 
 	/*

@@ -1053,8 +1066,6 @@ void __init setup_arch(char **cmdline_p)
 	 */
 	sev_setup_arch();
 
-	reserve_bios_regions();
-
 	efi_fake_memmap();
 	efi_find_mirror();
 	efi_esrt_init();

From: Mike Rapoport <[email protected]>

The early reservations of memory areas used by the firmware, bootloader, kernel text and data are spread over setup_arch(). Moreover, some of them happen *after* memblock allocations, e.g trim_platform_memory_ranges() and trim_low_memory_range() are called after reserve_real_mode() that allocates memory.

We did not observe corruption of these memory regions because memblock

always allocates memory either from the end of memory (in top-down mode) or above the kernel image (in bottom-up mode). However, the bottom up mode is going to be updated to span the entire memory [1] to avoid limitations caused by KASLR.

Consolidate early memory reservations in a dedicated function to improve robustness against future changes. Having the early reservations in one place also makes it clearer what memory must be reserved before we allow memblock allocations.

Quoted 8 lines · Mike Rapoport

From: Mike Rapoport <[email protected]>

The early reservations of memory areas used by the firmware, bootloader, kernel text and data are spread over setup_arch(). Moreover, some of them happen *after* memblock allocations, e.g trim_platform_memory_ranges() and trim_low_memory_range() are called after reserve_real_mode() that allocates memory.

Quoted 1 line

We did not observe corruption of these memory regions because memblock

Make that "We" impersonal, passive voice pls.

Quoted 5 lines

always allocates memory either from the end of memory (in top-down mode) or above the kernel image (in bottom-up mode). However, the bottom up mode is going to be updated to span the entire memory [1] to avoid limitations caused by KASLR.

Quoted 4 lines

Consolidate early memory reservations in a dedicated function to improve robustness against future changes. Having the early reservations in one place also makes it clearer what memory must be reserved before we allow memblock allocations.

Would it make sense to have a check with a WARN or so to catch early reservations which get added after memblock allocations have been allowed? To catch people who don't pay attention...

This would make sense but it's tricky. From memblock perspective, allocations are always allowed and it is the user responsibility to ensure all the early reservations are done before allocating memory.

So adding such a WARN would require a new memblock API and it's adoption by all architectures, which is way beyond the scope of this series :)

arch/x86/kernel/setup.c static void __init trim_low_memory_range(void) · line 760

Quoted 1 line · Mike Rapoport

arch/x86/kernel/setup.c +1 -0

+	trim_low_memory_range();

Btw, you can get rid of that one too:

/*
* Here we put platform-specific memory range workarounds, i.e.
* memory known to be corrupt or otherwise in need to be reserved on
* specific platforms.
*
* If this gets used more widely it could use a real dispatch mechanism. */
static void __init trim_platform_memory_ranges(void)

{
        trim_snb_memory();
}

yeah, yeah, we can do a real dispatch mechanism but we didn't need one since 2012 so I guess we can get rid of trim_platform_memory_ranges() and call trim_snb_memory() directly and simplify it even more.

Thx.

From: Mike Rapoport <[email protected]>

Currently the first several pages are reserved both to avoid leaking their contents on systems with L1TF and to avoid corrupting BIOS memory.

Merge the two memory reservations.

Signed-off-by Mike Rapoport <[email protected]>

---
 arch/x86/kernel/setup.c | 29 +++++++++++------------------
 1 file changed, 11 insertions(+), 18 deletions(-)

arch/x86/kernel/setup.c +11 -17

index 32cd2e790a0a..3f2fd67240f8 100644
@@ -715,20 +715,6 @@ static int __init parse_reservelow(char *p)
 
 early_param("reservelow", parse_reservelow);
 
-static void __init trim_low_memory_range(void)
-{
-	/*
-	 * A special case is the first 4Kb of memory;
-	 * This is a BIOS owned area, not kernel ram, but generally
-	 * not listed as such in the E820 table.
-	 *
-	 * This typically reserves additional memory (64KiB by default)
-	 * since some BIOSes are known to corrupt low memory.  See the
-	 * Kconfig help text for X86_RESERVE_LOW.
-	 */
-	memblock_reserve(0, ALIGN(reserve_low, PAGE_SIZE));
-}
-
 static void __init early_reserve_memory(void)
 {
 	/*

@@ -741,10 +727,18 @@ static void __init early_reserve_memory(void)
 			 (unsigned long)__end_of_kernel_reserve - (unsigned long)_text);
 
 	/*
-	 * Make sure page 0 is always reserved because on systems with
-	 * L1TF its contents can be leaked to user processes.
+	 * The first 4Kb of memory is a BIOS owned area, but generally it is
+	 * not listed as such in the E820 table.
+	 *
+	 * Reserve the first memory page and typically some additional
+	 * memory (64KiB by default) since some BIOSes are known to corrupt
+	 * low memory. See the Kconfig help text for X86_RESERVE_LOW.
+	 *
+	 * In addition, we must make sure page 0 is always reserved because
+	 * on systems with L1TF its contents can be leaked to user
+	 * processes.
 	 */
-	memblock_reserve(0, PAGE_SIZE);
+	memblock_reserve(0, ALIGN(reserve_low, PAGE_SIZE));
 
 	early_reserve_initrd();
 

From: Mike Rapoport <[email protected]>

Currently the first several pages are reserved both to avoid leaking their contents on systems with L1TF and to avoid corrupting BIOS memory.

Merge the two memory reservations.

Signed-off-by Mike Rapoport <[email protected]>

---
 arch/x86/kernel/setup.c | 29 +++++++++++------------------
 1 file changed, 11 insertions(+), 18 deletions(-)

arch/x86/kernel/setup.c +0 -14

index 32cd2e790a0a..3f2fd67240f8 100644
@@ -715,20 +715,6 @@ static int __init parse_reservelow(char *p)
 
 early_param("reservelow", parse_reservelow);
 
-static void __init trim_low_memory_range(void)
-{
-	/*
-	 * A special case is the first 4Kb of memory;
-	 * This is a BIOS owned area, not kernel ram, but generally
-	 * not listed as such in the E820 table.
-	 *
-	 * This typically reserves additional memory (64KiB by default)
-	 * since some BIOSes are known to corrupt low memory.  See the
-	 * Kconfig help text for X86_RESERVE_LOW.
-	 */
-	memblock_reserve(0, ALIGN(reserve_low, PAGE_SIZE));
-}
-
 static void __init early_reserve_memory(void)
 {
 	/*

@@ -741,10 +727,18 @@ static void __init early_reserve_memory(void)
 			 (unsigned long)__end_of_kernel_reserve - (unsigned long)_text);
 
 	/*

From: Mike Rapoport <[email protected]>

Hi,

David noticed that we do some of memblock_reserve() calls after allocations are possible:

https://lore.kernel.org/lkml/[email protected]

For now there is no actual problem because in top-down mode we allocate from the end of the memory and in bottom-up mode we allocate above the kernel image. But there is a patch in the mm tree that allow bottom-up allocations below the kernel:

https://lore.kernel.org/lkml/[email protected]

and with this change we may get a memory corruption if an allocation steps on some of the firmware areas that are yet to be reserved.

The below patches consolidate early memory reservations done during setup_arch() so that memory used by firmware, bootloader, kernel text/data and the memory that should be excluded from the available memory for whatever other reason is reserved before memblock allocations are possible.

The patches are vs v5.11-rc3-mmots-2021-01-12-02-00 as I think they are prerequisite for the memblock bottom-up changes, but if needed I can rebase then on another tree.

Mike Rapoport (2):
x86/setup: consolidate early memory reservations x86/setup: merge several reservations of start of the memory

 arch/x86/kernel/setup.c | 85 +++++++++++++++++++++--------------------
 1 file changed, 43 insertions(+), 42 deletions(-)

Signature · 1 line

2.28.0

Quoted 7 lines · Mike Rapoport

From: Mike Rapoport <[email protected]>

Hi,

David noticed that we do some of memblock_reserve() calls after allocations are possible:

Quoted 1 line

https://lore.kernel.org/lkml/[email protected]

Thanks for CC-ing me, so I think the above patch from Roman is dangerous. KASLR does put kernel randomly in a place, but we did a brutal parse to get SRAT table so that we know where is hotpluggable area during boot decompression stage. In kernel, at the beginning, we don't know that before ACPI init. Roman's patch is wrong if I don't miss something.

I will add comment in that thread.

Thanks
Baoquan

Quoted 30 lines

For now there is no actual problem because in top-down mode we allocate from the end of the memory and in bottom-up mode we allocate above the kernel image. But there is a patch in the mm tree that allow bottom-up allocations below the kernel:

https://lore.kernel.org/lkml/[email protected]

and with this change we may get a memory corruption if an allocation steps on some of the firmware areas that are yet to be reserved.

The below patches consolidate early memory reservations done during setup_arch() so that memory used by firmware, bootloader, kernel text/data and the memory that should be excluded from the available memory for whatever other reason is reserved before memblock allocations are possible.

The patches are vs v5.11-rc3-mmots-2021-01-12-02-00 as I think they are prerequisite for the memblock bottom-up changes, but if needed I can rebase then on another tree.

Mike Rapoport (2):
x86/setup: consolidate early memory reservations x86/setup: merge several reservations of start of the memory

 arch/x86/kernel/setup.c | 85 +++++++++++++++++++++--------------------
 1 file changed, 43 insertions(+), 42 deletions(-)

Signature · 1 line

2.28.0